I think it is strange update. Our sites are
PR 6-> PR 6
PR 6-> PR 5
PR 5-> PR 6
PR 5- PR 5
All that I can say that this site PR 5-> PR 6 was aproximatively abandoned, very few links added. I don't know the reason of raising PR.
As we analyse our backward partners- 14 of them dropped from PR 6 to PR 0. Though #*$! still shows some PR 0, some PR 6. I don't know the reason of it too.
We noticed such "banned sites" after every backward links update, so "no" to the theory of quarterly Pr update?