You're looking at it in the wrong way. Someone with Engineering skills could improvise some of the skills that mechanical and repair includes, but they wouldn't be trained in it. The skills list is for things that you are effectively trained in, not just a list of the ways you could possibly twist your skills to fit. It is, in theory, possible to use the biology skill as a rudimentary medical skill, but a biologist isn't the same thing as a doctor, and vice versa. Just because skills tie together doesn't mean they're intended to be subsets, it just means they work best hand in hand. An Engineer with Mechanical and repair skills will be much more effective than one with just Engineering.