In a fan site I did find a comment that various equipment items get a partial bonus. This probably explains it, though the item description should be updated!
umm.. you dont get full value for the smaller piece of gear. When you select the enchant.. IT SHOWS YOU A PREVIEW OF THE OUTCOME>.. of you dont like it.. dont hit the 3 KEYS IT TAKES TO ACCEPT THE ACTION.
I don't know if this was your case, but you get smaller benefit from +magicka than stated on the tooltip if yor magicka stat hits the soft cap. You can see it on the stat screen, if your magicka stat is orange, with a lightning symbol next to it, the stat passed the soft cap and any further increases become less efficient.