Normally, I'd just say that it's the riding position (assuming that the riders' fitness is about the same). The same rider on different bikes can go faster, or the same speed with less effort, on a bike that lets them use a more aerodynamic position.
But, OP, since you mentioned "the same gear", you're unwittingly bringing up the different gearing usually seen on these bikes. Mountain bikes aren't meant for chugging along on a paved road at 25 mph, just like road bikes aren't made to climb over tree roots at 5 mph. The gearing for each is tailored to the task.
My three bikes -- coincidentally one each of road bike, hybrid, and mountain bike -- all have different gear ranges, and they're just as others have described already. And, even though I'm running the same 50/34 chainring combo on both the hybrid and road bike, the hybrid's cassette is a little wider and gives me a lower low gear.