JPEG or RAW, all my digital Nikon bodies for the past tens years have been conservative in the frames remaining. Since each image file compresses or saves differently, I'm guessing that the frames remaining has to do with worse case scenario.
Imagine, if the counter said 100 and only give you 80 frames, there's be lawsuits a flyin' here in the U.S. that the camera owner was duped. But, by going the other way the typical consumer can feel he's duped Nikon with all the "bonus" images! <G>
Once you start filling the cards in real life, you'll be able to get a feel how many frames you'll normally get from a card. For me, it's roughly a 50% increase in what the remaining frames show, though I've never actually counted and don't really worry too much about.