Yes. Your barrel and scope aren't parallel. They have to converge slightly, as the pellet has to rise to intersect with the line of sight through the scope.
Depending on how high above the barrel you mount the scope, you need more convergence.
A fixed rail adds a fair bit of extra height, so some angle is usually built in. It tips the front of the mount downwards a shade.
If you didn't have this built in, you'd have to try to take it up with adjustable mounts (££) as you'd probably run out of adjustment on the top turret. In any case, you want the top turret at roughly the middle of its travel rather than at one end, to get the best out of the optics.