Have you actually seen critters spawn next to workbench? If so, send a bug report. If not, then indeed you might have a sliver of ground uncovered. If some of your coverage is from above ground, like on a second or third floor of a building, the coverabe is like a bubble, so it will be smaller area as it gets lower (if that makes sense - think of the item as the center of a ball, not just within a simple circle, the diameter changes both above and below).

The best way to check coverage is to run around with a small floor or wall piece and look for the problem. It really can be a very tiny area. You can also validate files in case there is a corruption.