Because they've always done that, since TOS! The Galileo shuttle is said in dialogue to be 24 feet long, but the exterior mock-up is only 22, and the interior set is too big to fit inside either!Come now, why would they scale the ships to one size on the show and then give us a different size?
No Trek production has ever consistently adhered onscreen to the scale(s) established for its ships offscreen. They go with what looks dramatically and aesthetically pleasing for a given shot every time!
And I know you know this as well as I do, because you used to be the one pointing it out here at every opportunity you got. What's up with that, man? I don't grasp your game in all this. Help me out. I'd honestly like to understand where you're coming from. Are you just trolling? (Not an accusation, a genuine question.)
-MMoM
