Made my own investigation! Reason and solution found!
Turned out to be APPLE BUG! Buggy PC client didn't delete Turned off Icloud Photos from occupied space.
Cured by turning on and off again apple photos in ICloud Storage Manage tab. See below.
Look here (piture below, iCloud -> iCloud Storage Manage). Photos in iCloud Photos are stored for 30 days after turning iCloud Photos off on ALL your divices. Yet, space occupied by them shpuld be reported empty right after u turn them off on all your deviced. Yet, the bug in PC client reportd iCloud Photos storage OCCUPIED while it should not!
Now this is corrected by making the same operations on Mac and iPhone
Long story short: due to some bug in PC client iCloud Photos when turned OFF continue to report their space occupied for 30 days more while they should NOT (while they still there, for 30 day free). To fix if - tweal Photos tab in iCloud Storage -> Management. On iPhone or iMac.
PS. Storage freed 2 days after tweaks in iMac! Not immediately.