The main grammatical difference, which hints to the usage difference, is that 些 is indeed a classifier.
This implies 些 gets involved when you're counting stuff. Now you say: but 些 is supposed to mean "some", I'm not really counting anything!
Well nevertheless you are, even though you are not expressing a precise quantity.
Now, let's add a piece to the puzzle.
As you may know, classifiers in chinese are also used to determine a noun, i.e. they replace the determinative article in English. That's why 那个苹果 can be translated as "that apple" but also "the apple", that one apple we talked about earlier in the conversation.
Our 些 in turn carries the same semantic value.
一些苹果 some apples
那些苹果 these apples / the apples
Therefore 些 is used when you're talking about a narrow topic, something circumscribed to a pool of common knowledge between you and your interlocutors.
Whereas 有 means "there is" and 有的expresses a general existential trait, as in 有的人喜欢吃苹果 = "there are people who like to eat apples" = "some people like to eat apples".
些 particular, unspecified but possibly finite number