There are two factors for the rack & pinion gear type (primarily Power Gear brand slides). One is that the front and rear gear tracks may be skewed by one or tow teeth, cocking the slide slightly. That skews the whole slide by maybe 1/4", but the bottom normally contacts the sidewalls before the top, so the top is where a gap shows. The second is a set of adjustment bolts/nuts at the outer end of the gear track under the slide.
I'd say there is a fair chance that the slide jumper one tooth on the gear track, either originally from the factory (quality control is minimal) or through abuse of the slide at some point. If the slide is moved in/out with something in the way that jams it, the motor-driven gear can jump a tooth on the track (gear rack).
