Is it hydraulic or motor/cable and pulley. If the latter, then there is a chance the cable is out of the pulley some place and one side is 'longer' than the other, causing one edge to be slightly behind the other.
The one on my fiver is screw drive with a rack and pinion synchronizer. When it goes all the way out, the plywood floor drops down to be even with the main floor. So as it starts in, it tips up a bit. As it comes in further it tips up even more. Eventually, it passes a balance point and tips back down so that the slideout floor is parallel to the main floor. But to make sure the top seal seals out all of the rain, the outer wall of the slight is deliberately canted inward slightly toward the top. So as the top of the slide touches the main wall, the floor of the slide starts tipping up again. But it only tips up 1/4" at most.
My new TT on the other hand . . . well that slide is all messed up and jammed up. That is why it has been at the dealer for almost 2 weeks now. They need to reroute the slide motor cables and move the slide controller in order to fix it right (Schwintek/Lippert system).