Question Why Are Lights Ignoring Their Culling Masks

I'm using baked point lights to light my rooms. I have their culling masks set to light everything but the ceiling, which is in its own layer. The ceiling is set to neither cast nor receive shadows. I'm using the Progressive GPU light mapper, baked global illumination is on (subtractive), realtime global illumination is off, ambient occlusion is on. I have two additional realtime lights in the scene, but they only light non-static objects, which are in their own layers.

So why am I getting hot spots on the ceiling? The only way I've been able to get rid of them is to lower the lights' range or intensity, or move the light, neither of which is acceptable.