Because it was not a cutting from a Plum tree. Plums are Prunus and Apples are Malus.
Also neither grow easily from cuttings.

might have been plum on an apple rootstock. the cutting would have been from the apple rootstock.

Sorry but you cannot put Plum on an Apple rootstock, they are such different species. Be like grafting a pig's foot onto a human leg.

Mistakes happen. The cuttings I took from fuchsia Alice Hoffman last year turn out to be 'Tom Thumb...

I'm surprised to hear that you grew a cutting from either a plum or apple tree. As Owdboggy said, it's very hard to grow them as a cutting, so if it took, then well done.

Also, as Owdboggy quite rightly said using his analogy, it's impossible to grow a cutting from a plant species that will turn into a different species, so... You must have propagated an apple cutting instead.

If you managed to get it going from a cutting and then got it to produce fruit, then it will be an exact clone of the donor plant and you have done a great job.

I have taken suckers from Apple rootstocks and got them to grow. Of course they just grow into what ever the rootstock was. Only useful to graft pieces of cultivars onto.

I didn't know that owdboggy, thanks. I thought I'd read some where that the dwarfed rootstocks were all Malus. perhsps it was for apples. I read so much stuff some is bound to slip out with the jelly!

It was definitely a cutting from a plum tree as it was from my dad and he never had an apple tree only plum and although it may be difficult he did manage to make it grow Green fingers

If you tried to graft an apple tree scion onto a plum rootstock it wouldn't take. Yes, they are both fruit trees but from different families, one is from the Malus family, the other Prunus. Cats and dogs are both mammals, but you cannot cross one with the other, one is canine, the other feline, it's against the rules of nature.

So to get apples to grow on a tree that was a cutting from a plum tree, well, that's totally impossible.

Your dad must have planted an apple tree cutting.

Well Anita you must be feeling pretty frustrated by now.
Better buy a plum tree now if you want plums!

technically Malus and Prunus are in the same family but they are different genera.
Plant cross breeding obeys fewer rules than with animals.
Fatsia and Hedra are in the same family and when crossed you get Fatshedera. as we know there are some wonderful plants from these sorts of crosses.