A level 40 Aegis reduced your casting total by 20. This aspect shouldn't have any effect on an item, since items don't have Casting Totals. Where it affects items is by them being Resisted by the Aegis, which I'm reading as having to overcome the Aegis as Magic Resistance, i.e. Penetration. I'll have to check the handy-dandy Aegis Cheat Sheet, though.