Essentially, it all boils down to the fact that you don't have separate x terms and y terms. Rather, you only have xy terms. I think it might be easier to see using real values. Let's say x = 2 and y = 3 and re-examine the given equation.
4xy + 2xy = 6xy --> 4(2)(3) + 2(2)(3) = 6(2)(3)
I see your question as asking: 'I've got four "2" terms and two more "2" terms, so I've got 6. And I've got one "3" term and one more "3" term so I've got 2.' But that's not really what's going on here. You don't actually have any "2" terms or "3" terms. Instead, you only have "6" (2*3) terms. And you have four "6" terms plus two "6" terms, so you have a total of "6" terms.