The problem with quotas is that the feature itself isn't yet mature. At least until very recently, and possibly still, quotas couldn't be depended upon to work correctly (various not entirely uncommon corner-cases would trigger negative numbers, etc), and even when they do work correctly, they simply don't scale well in combination with balance, check, etc -- that 10X difference isn't uncommon. Understanding the code in Btrfs quotas, give a analysis & improvement.