I used PlanAhead to create PBlocks for my design. PlanAhead showed all my PBlocks to be under 100% utilized, but when I run ISE Place and Route, one or more of my PBlocks is overmapped.
PlanAhead gives pblock utilization based on post-synthesis estimation. While this is generally close to actual utilization, in some cases it may not be completely accurate. The map results are the definitive utilization results, so pblocks will need to be made bigger if they are overmapped.