Volumetric Lightmap Doesn't Match Static Lightmap

In the attached image, the highlighted box is using dynamic lighting with volumetric lightmaps, and the object behind it is prebaked indirect lighting in a normal lightmap. For some reason the static object seems to be getting indirect lighting that should be occluded, while the volumetric lightmap is more accurate with the bottom of the box appearing darker.

These were baked using high settings and all the rest of the lightmass world settings at default. Does anyone know what is going on here? Is there something I am missing that needs to be done to get these to match? Thanks for any help.