Not sure what it means, other than it just seems to be typical behavior for them. I guess I don't notice in my tanks with rims because they all have covers and I just don't see it.
On another note: One of my oldest PFR females (at the moment) just died. 599 days old - almost made it to the 600 mark. She only had two clutches and lived about half her life with some molt-related deformities. Quite a record for this blood line.