Stat gpu vs stat unit

I’m optimizing a game that contains a whole bunch of procedural meshes.

The gpu time given by stat unit differs significantly from the total value indicated by stat gpu if I set r.HZBOcclusion to 1. On the flip side, if that value is set to 0, stat gpu values start to match the stat unit gpu time.

Are there any additional settings I need to use for the HZBOcclusion to actually work?