Pile direction is the whole problem
Velvet and chenille are constructed from raised fibre standing off a backing, and that fibre has a direction. Light reflects differently along the nap than against it, which is what gives velvet its depth. Introduce moisture and the pile becomes temporarily moveable; allow it to dry in a disturbed position and the new direction sets permanently. The result is a sofa with patches that look lighter or darker than the surrounding area in daylight, and no amount of subsequent cleaning corrects it. The fabric is not stained. The fibres are simply lying the wrong way.