why Lightmass Importance Volume can affect lightmaps

From documentation of UE I learned that Lightmass Importance Volume(LIV for short) is used for dynamic objects, providing Spherical Harmonics for them. And I always thought that LIV is baked after normal lightmap. But our TA just made a discovery that LIV do have an effect on lightmaps of static objects:

The first photo is the scene without LIV, and the second is with LIV wraps the whole scene. The second is much brighter than the first one.

All model in this scene is marked as “static”, there is only one directional light marked as “movable” and one point light marked as “static”, no skylight is used.

It’s wired that LIV makes my “static” models and lightmaps brighter, why should that happened?
I guess UE is baking lightmap and LIV in each indirect lighting bounce calculation, and use this LIV for each static objects for the next lightmap generation. But it’s just my guess, no prove and documentation for that. Is my guess correct?