One possibility is a temperature difference between when you cut the part and when you measured it.
Another is that there were stresses in the acrylic and when you cut it this allowed the stresses to relax causing the part to distort.
When machining acrylic sometimes it is a good idea to stress relieve before machining and again after machining. Search on Google using 'annealing acrylic perpsex plexiglas' and you should find something.
Also try to always use cast acrylic when machining not extruded acrylic; the cast has less stress in it.