Yeah, think of volume voxels (and VDB) like something built out of blocks, pretty much like some structure in Minecraft.
TG on the other hand uses its internal noise functions and can calculate them at pixel level, and uses voxels only to compute light propagation and GI within volumes. The fact that we can export VDBs is basically a side effect of TG using voxels.
At least that's how I understand it, judging by all my tests and VDB in other apps I guess this is more or less what's causing the difference.