Sounds like when they built the house, they framed the interior walls afterwards. They build them short so they can stand them up inside, then shim them up to hold them in place firmly.
Not a good practic, but as long as they are not load bearing, it does not hold much weight.
Another reason is that the house floor and ceiling joists also shrinks over time, but the studs do not shrink lengthwise.
Screw the floor, and it should be fine, good luck.