DBC.DiskSpace is a view based in tables DBC.DataBaseSpace and DBC.dbase. These tables account data blocks used by database objects, therefore they do not account the space within the allocated cylinders that are not used.
But TVS considers the cylinders allocated per AMP, regardless of their utilization percentage.
TVS migration will move cylinders as they are, without pack/unpack them and to work efficiently TVS migration requires a minimum amount of free cylinders, under which it get disabled.
In TD 15.10, TVS migrations are stopped when allocated cylinders in an AMP reaches 95% and it will not start again until all AMPs, assigned to the TVS allocator, are having allocated cylinders under 93%.
Migrations stop/start in the TVS vproc where the AMP, that reached the threshold, is assigned.