crab apple grafted onto regular apple?

It seems that one of our crab apple trees was grafted onto root stock of a regular sized apple. The tree is covered with crab apples and a few large ones. I can't think of any other explanation.

Has anyone heard of this before?

The grafted in limb could have died out. I would graft in a new limb from and apple tree you like, then (over time) prune out all of the current crab apple producing limbs.

Agree with the above, but you might want to leave one small limb of crabapple to help with cross-pollination.