Why is it that my Invisalign trays appear to change shape after wearing?

I have a functional posterior cross-bite that may or may not be completely corrected at the end of treatment. The trays have some expansion in them. When I fit the top and bottom trays together before each first wear of the new aligner, they are not in cross-bite. When I do so after wearing them for a while, the trays appear slightly in cross-bite. Is this change in the tray arch typical?