That's the way mine are too. The lower extruded aluminum piece is solid but the plastic cover is in 3 pieces. On the front of both drivers and pass side the front short plastic piece sticks up further than the joining middle piece. Looks absolutely terrible.
If your plastic pieces are moving around, you probably have broken a tab or somethin. The aluminum step base has holes in it that allow the tabs from the upper plastic covering to locate into. This keeps the plastic in place and snapped down to the aluminum board. If any of your pieces can move i would suspect that you have a few tabs broken off the plastic cover. That's about the only way the plastic pieces can move on their own