This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
1. The blade may have shifted sideways slightly when the front or back
pair of screws were tightened on the temp mounts, warping the blade
from front to back.
2. The holes for the permanent mounts might not be positioned
perfectly, warping the blade as in #1 above.
3. The heel might not be perfectly level or flat with respect to the
front of the boot. Old screw-holes may have created bumps on the heel.
Or the boot might have been manufactured with an uneven heel. Such a
heel will twist the blade.