You're talking about the Viper, right?
We constantly forget that the Viper is a multi-environment fighter craft.
It is designed to function in atmosphere as well as the vacuum of space.
It is amazing how many people point out the wings and get all uptight about them because you don't need wings in outer space. But the fact remains, you DO need them in an atmosphere... which is one of the main functions of a Viper... operate and launch from planetary surfaces.
The turbo-fan details you see could be a portion of the "atmosphere motor". There's nothing in canon that indicates they didn't use two completely different pusher systems to motivate the craft in different situations. In fact, it kind-of makes sense.
OR it could very well be that since this is an advanced technology you are only assuming they are turbo- fans when they are actually "bussard collector units" (sp?) or "anti-burn baffles" or an number of techno-jargon items. Just because a lemur has a ringed tail and a dark mask doesn't mean it is or is related to a raccoon.
If it looks like a duck and quacks like a duck then it MUST be a duck... unless you're in a television sci-fi show and then it's probably just your infinite improbability drive acting up.
OR it might just have looked cool. You never know with these TV designers.
-Gordon