What's happened to my crab apple?

[Redactio]

Hi:
I have a small crab apple tree (i use the fruit for jam making). This year the bottom branches have normal crab apples, but the top branches are bearing fruit that are apples, but definitely not crabs.
Any of you gardeners know what might have happened?

Is it grafted onto an apple root stock and that has come through?

I don't know, that may be the answer. The tree is over 5 years old and I can't remember where I bought it from, it was one of the big commercial fruit tree suppliers.