Something about the room may not be entirely "square". That, I gather, is more common than it should be.

This problem is common when people install blinds or shades on windows. There can be a difference across the space from top to bottom. When measuring for blinds one should measure across the top, middle and bottom. It is also advisable to measure every window because there may be only one where the opening is problematic. Otherwise one could find a blind binds against the trim at some point.

Framing lumber is not necessarily straight. Each piece can have a crown, bow or cup. A poorly placed crowned board could cause a bulge or dip in wallboard. A wall that tees into another can also be out of PLUMB or out of square to the wall it butts against. It happens and the trim carpenter has to make the defect disappear. No discrepancy was noticeable until you added the trim.