Another Rattling Sceptre bug?

Increasing Rattling Sceptre quality is decreasing Skeletal warriors damage.
On my Sceptre quality 19 Basic Attack tab was showing as follow:
DPS 189.84
Damage per use 159,53
Attack Time 0,84s
Attacks per second 1,19
after increasing to 20% stats changed to:
DPS 188,25
DPU 159,53
Attack Time 0,85
Aps 1,18

I've done some more testing and look like increasing sceptre by 9% make stats to go back to base and after that start decreasing again. Improving quality according to tooltip window should increase skeletons hp but there is NOTHING about decreasing their DPS what is making whole quality process basically pointless.