Its really hard too say with this one. Here is another one of those Inconsistancies that occurs in Star Trek. That is why I always liked TNG and DS9, the inconsistancies where at a minimum.
As for the Miranda being a Surya, its very possible. the Regestry number is not much higher then the Connies so that theory could easily fit. And we all know that their was only 12-14 Connies built during the TOS era, but that doesn't nessessarly mean that Starfleet never built anymore then that. I suppose that Starfleet could have easily just copied the whole Connie design (after the Refit of the Enterprise and other suriving Connies in the 2270's) and remanufactor the class again with updated systems. The only problem I have with that is just by the fact that in ST1, it seems that the initial design was pushed too its absolute limit in capabilities. Ya, ST5 and ST6 shows that the Constition 2's has some vastly different internal restructuring then the Refitted Connies of the 2270's, but it seems that overall the Connies never really got any more then minor additional Capabilites. That is why I suggested that Starfleet might have considered a new Crusier design too replace the Connies. The Constellations could be a very good canidate for this as it seems Starfleet jam-packed a lot of additional systems into the Consellations (4 warp engines, much more impulse power, lots of scanner devices, lots of additional Phaser mounts and possible extra Torpedo mounts, etc.), or its possible another type of crusier was developed too have as a "go-between" for the planned Excelcior Class and the Mirandas.
I personally have a theory why we see sooooo many Mirandas in the TNG era is possibly due too ship yards within the Federation. I also think I may have an explanation in why the Regestry number is so close too the Constitions. One possible theory on why the Reliant has such a low Regestry number is that it originally was a Surya at one time but was heavily refitted just like the Enterprise. If I recall from our own navy, Ship production is planned well in advanced with plans, hull regestries and even ship names already preplanned well before the keel of the ship is even layed out in the ship yard. Do you realize guys that our US navy already has on the books 30 new destroyers (forgot the class name) that will star construction in the next couple of years? It will take rougly a decade too get all 30 constructed but they are designed with the latest technology and are planned too be in use for the next 40-50 years and are planned too replace a lot of the old destroyers in our navey that where built during the 60's and 70's. I think Starfleet might have had a similar approach. According to the TOS tech manual, it shows a DN, the Connie, and a DD/scout (the Salidin Class). Its possible that Starfleet contracted somebody too start making 4 type of ships back in the early 23rd century and all would be built around the same time. The DN, the Connie, the Surya, and the Salidins might have been all pre-planned well in advanced and the larger the ship, the less would be made. For example, Their would only be a handful of DN's, about 15 planned Connies, maybe 50 tops of the Suryas and possibly 80-100 Salidins. As we can see, all these ships seem too have common components which means that they possibly all came from the same shipyards. When the Refit stage came buy for these ships (2270) whatever ships that where left of this run of ships, were refitted with the new Technology. The Reliant Might have been one of these Refitted ships and why its regestry number is so close too the Constitutions and so low. During and after this refit stage, Starfleet might have ran into an economic and logistics problem. Starfleet did a remarkable job refitting the old vessels over too new Tech, but a need for new Ships too fill roles/losses would have too be made. Granted, Starfleet could have just copied the Post-refit designs and run carbon copies of the design with bran new ships (aka, new refitted Connies, Suryas, Salidines) but its possible growing threats from Klingons and Romulans and advancing technology forced Starfleet too rethink new ship designs. The Connies where great ships, but cost a lot too build and took some time too construct aswell (the DN's could fall into this catagory aswell). The Salidines were very cost productive as they were small and cheap too produced, but hampered with design problems (lack of warp speed, low power for weapons and systems). The Suyras (Mirandas after the Refit stage) where the only designs left that could take advantage of both being fairly cheap too produce in large numbers, and have some capabilities as a Connies would have. So if Starfleet was too scrape the initial designs of the other three ships and keep producing Suyras/Mirandas, this frees up a lot of ship yards to concentrate on building Mirandas, while Starfleet could design a new ships too fill roles. The Its possible the Excelcior concept was started not long after the Refit stage, and Starfleet might have wanted this ship too be a type of Dreadnought or possibly a "Super Crusier" too fill that role. The Mirandas could fill the Destroyer role for combat purposes, but Starfleet would need too have some sort of crusier available too fill inbetween the Mirandas and Excelciors. The Constellations could very well be this ship (mabye another class we never saw). The ships is loaded with systems that looks too outweight the Connies. Its possible Starfleet wanted too use existing technology (as it would be cheaper then using Advanced excelcior tech) so that the class could easily fill the role of heavy Crusier with lots of capabilites (the "Poor mans Crusier"). Not only is using existing tech a cheap alternative, but since many of the components are very similar too components used to build Mirandas, it would be easy too use one of the Miranda ship yards too build Constellations.
Holy Cow! That was a lot of crap I just wrote. Keep in mind people that this is just a hypothetical theory and I'm not referencing anything. It just seems too me that the Mirandas where a fairly cheap vessel too produce and one way too make a vessel cheap too produce and build high numbers with is too free up a lot facilities that would build other vessels. And the Constellations seems too use many parts that Mirandas uses so its logical too assume that the Constellations where somewhat easy too produce too (at least compared too a sophisticated Connie).