Actually I tend to go with the far easier "ILM didn't give a hoot about the registries" route.
The FC designs were only intended to be background ships (as clearly evidenced by the low quality nature of the models in terms of both textures and polys - the slight exception being the Akira*), so I imagine they gave very little thought to it.
Oh, I totally agree with you about ILM...I truly believe that in STIII, they gave the Excelsior a registry of "2000" just because she was a big ship, while the Grissom only had a 638 registry because she was small
Furthermore, several of the design cues present are similar to Defiant (see the Norway-class) which I'd think we should all be able to agree was pretty much a first among Starfleet designs.
Exactly. The Norway's ventral view (which was simply a cut-and-paste jumble of the Defiant's texture) is another example of why these ships were new as of FC and not older per a chronological registry scheme.
Of course, there's always the possibility that the background ships looked substantially different when they were first commissioned, and that the styles seen in FC are indicative of heavy refits and modifications - likely in the wake of Wolf-359 itself (it's not as if we don't have precedent for it, what with the Constitution refit... and I also adhere to that rule for the low-registry Grissom seen in TSFS)...The Akira I regard almost certainly as having had a partial refit, what with the saucer, registry, clunky nacelles, old-fashioned deflector.... and then the new-fangled lifeboat hatches slapped on top.
See, I kinda have a problem with the idea of the FC ships being refitted from older ship designs. Mainly because we saw a ton of even older
ship designs still in use in TNG (the Miranda, Excelsior, Constellation and Oberth), and none of them have been refitted to look any different from when we saw them in the 23rd century.
Plus, the majority of the Starfleet vessels used in the Borg attack of Sector 001 were the FC ships, implying that these were the new vessels Shelby mentioned in BoBW when she said the fleet would be back in less than a year. They were built to be Borg-killers, which was why we saw them, uh, attacking a Borg ship
I think the whole "refit" thing is analogous to the whole "Klingon blood is Pepto-Bismol pink" thing: It was used for the film it was portrayed in, but was never touched upon again after that in future incarnations of Trek.