My girlfriend told me that when she feeds Deer apples, the Deer won't eat the green Granny Smith apples. Anyone know why the deer won't eat those?

The only reason I can think of or even find any info is that green apples take longer to ripen and before they get a little softer and almost over ripe, they are a little bitter and starchy. But I've not ever seen a deer that won't eat almost anything.