That;s because there were at least TWO physical models made for the ship to film TNG.....for the first season we had the 10-foot long "sleek" ship which had the aztec panelling only painted on, but for season 2, starting with that episode about the supernova, the scientist, Wesley and the nanites (dang, can't recall the title) they used a 4-foot long model that had different details, and RAISED aztec hull panelling. If you notice from that episode onwards, shots of the ship become darker as it should be in space if you think about it....whereas for all of season 1 when they used the 10-foot model the ship was way too brightly lit from all around.
Yes, so both the Ent-D and the Ent-E recieved "upgrades" in looks after the first season or movie, but for the Ent-D footage of the original 10-foot model was recycled in episodes throughout the remainder of the series, like they did for TOS when they had 2 different models, a small one and a big one, and the big one went through changes starting with the episode "The Corbomite Maneuver". Confusing eh? Ahhhhhh.........we're Trekkies, we're used to it.