As you probably know, plants use external cues to establish the correct seasonality of their various processes (leaf senescence, dormancy, flowering, fruiting, etc…). If something happens to throw this off, they can easily flower, and sometimes even fruit out of season.

After a major August hailstorm at my place a few years ago for example, I saw many trees resume growth as if it were spring again. They grew new leaves and shoots, flowered, and fruited (in the case of the mulberry) before killing frosts came in October.

Similarly a few years ago a record cold snap in late winter killed the flower buds of all the fruit trees where I live. There was no bloom that spring, but by July, August, and September, various types of fruit trees were beginning to flower sporadically.

I think I read somewhere that Granny Smith does not have fruiting spurs but blooms on new growth? I have an apple book but Granny Smith isn't in it for some reason? It's semi-dwarf but probably 12 foot tall and growing rapidly. I've tried pruning it lightly for shape. I'll try to post a picture of the whole tree soon.

Here in California the fruit will stay on the tree until after Christmas. It lightens up and gets much sweeter as it ages on the tree.

I would suspect that you aren't getting good pollination for some reason - either too few Bees or not having another tree nearby. While GS trees are self fruitful, it always helps to have another tree that can contribute.