It's not supposed to, and that's the *only* difference (because the Nebula had hidden impluse engines). Other than that, the in game model uses Galaxy/Monarch parts otherwise identical to the Galaxy/Gal-R.

As I remember things:

1. Game comes out with 3 Galaxy Variants. Cryptic says "we couldn't make the saucer seperation, and we'll probably never will." Aside from that, the model has tons of modelling issues.
2. Some point later, the Galaxy-X appears as a unique special reward if you refer 5 players. At this point, there was no plan to it become a "common" ship availlable from the C-Store. The ship was pretty much a special deal only, and possibly because it was a special reward, and CapnLogan was more awesome than the ship designers that preceded him, he put more attention to detail. It may also be possible that they had engine improvements done or analyzed their customer base to find ways to put in more detail work in new ship models without negatively affecting game performance.
3. Miracleously, members of the Cryptic team figure out a way to make Saucer Seperation work. As a result, we ultimately get the first 3 Retrofits, as reward for becoming Vice Admiral (character unlock) or via C-Store acquisition (account unlock). Making saucer seperation work required a new model entirely. Making it work with the Envoy and Celestial required basically the same overhead, and since I presume neither model was really liked by the community or maybe just the team / CapnLogan, they instead created a second, new variant. (Almost the same work, but with the advantage of actually being something new and looking new), the Monarch.
4. Over the past time, CapnLogan did a lot of work to fix various model errors that the community spotted out for him and demanded fixed.
5. End Result Probably: At least 3 Different Galaxy Class Models, with one having beein constantly adapted and improved. For all intents and purposes, the Exploration Cruiser, the Exploration Cruiser Retrofit, and the Galaxy Dreadnought are 3 different ships, as dissimilar in many ways as the Science Vessel and the Heavy Escort.

If someone had done a good job at the start, and maybe Cryptic would have had 6 more months, maybe all this could have been avoided. But that wasn't the case, so...

Do it.

Zephram "I always liked a good peepshow" Cochrane