Since you are trying to come up with possible explainations for things, if R2 and 3PO were known very well by Obi-Wan, why did Obi-an not recognize them in A new Hope when Luke was recused by Ben from the Sand People? Huh, huh? Explain that one all knowing J. Carney! LOL
In the book, Obi-Wan displays a little more depth, and that depth includes a trait that the original character in the first trilogy displayed, almost in spades- Droids are just objects. They are tools, and certianly useful ones, but nothing to be overly concerned about.
I mean, you don't remember every different firearm that you've handled in the course of your life, or every wrench, rachet, socket, or attachment that it takes to work on the engine in your car.
There are kids that you went to school with for a decade that you smooth forget 25 years down the road.
That's what happened to Obi-Wan. Oh, I'm sure if you poked and prodded and made him dig around in his brain he'd remember that Anikin always had this opinonated, loud-mouthed astromech that he treated like a sentient being which constantly followed him around... but I doubt that he could pick
IT out of a lineup, anymore than I could pick my first M-16 out of a lineup.
There's your logical explination...TAKE THAT!!!