Why does my CoC still cost 100 spirit even though at 20% quality?

If you socket it in does your spirit decrease by 100 or 91?

You don't look where you should...go to advanced info

Have you tried it with socketed gems? Maybe it's a visual bug, or simply it's not showing the right number like a lot of other tooltips

If you put it on and look in the details of an equipped gem it will show the effective cost there.

Is it really costing 100 or just a bug in the tooltip? In general I think that the reservation on the top is simply showing the base, if you try to activate it, is it taking 100 or less?

Sadly, aside from that I would call it a bug, or you would have to have reduced reservation on your gear/tree that would counter the 10% you get from the quality

Does increasing quality lower the spirit cost of spirit gems?

It needs 100 spirit to equip but uses 90-91 actually.

It is same with equipment with attribute modifiers. Like you have a glove that gives 20 dex and needs 80 dex to equip. You can't equip it with 60 dex but if you equip it with 80 dex you can change 20 of your attributes and you should be fine