Knowledge can be learned but aptitude is more or less defined in terms of not being trainable. Of course, this might mean that it's simply defined out of existence, but my experience has definitely been that it's much easier to teach some people things than others. Indeed, I experience a significant conflict between helping students who're most at risk of failing, and thereby accomplishing very little actual instruction, because they're mostly so difficult to teach, and focusing on the students who could pass under their own initiative, who'll actually absorb and comprehend the instruction, but can get by without it.

There are issues of attitude as well as aptitude, and they're closely intertwined, but they're not the same thing, and when you deal with a lot of people who vary along both metrics, it's hard to avoid differentiating between them.