Support Gem Capacity & Weapons????

I have two different quarterstaffs. Neither one of them have any strength/dex/int buffs on them, but when I equip one of them, my Support Gem Capacity drops, and some of my skills are unavailble. They show up in the skills menu with red exclamation marks next to it.

I don't get it. Why would changing Quarterstaff's affect my skills like this????