How does this bug conjuration? I don't understand?

Not only does it have a chance to absorb enemy spells it also has a chance to absorb spells that you cast on yourself.

Why would it have a chance to absorb spells you cast on yourself?

Because it's bugged......

That got past testing?

It's not so much bugged but just that it's exactly how it functions: It adds to your chance to absorb any magic cast on your character, and pretty much all conjuration spells are considered as being cast on self.

What is intersting is I have never had a Alteration armor spell fail because of it but I have had Conjuration spells fail.