Degree of ripeness - sun exposure - level of humidity on one side of the tree may be higher if shaded.

Another possibility is the apple variety is a hybrid and some apples are reverting to one of the parent species - don't know if that happens with apples but it does happen with flowers on occasion.

I agree with Kay. It Sounds like a hosting problem.

Have a Great Day!
Michael

Growing up the father of my best friend ran the Winchester Virginia Apple Parade every year. That would be the closest I can come to an apple expert.