Oh, I know the answer to this one! I think. It's because when you cook with a lemon you basically only really want the juice so at it's most basic, it's practically water! So it ends up looking a lot lighter then you would really expect.
On the other hand, you have to mash up the banana--although I really don't suggest you do that while it's still wrapped up in its skin--so you end up with a banana paste. But, because bananas turn a darker color after it's been peeled, anything that uses it tends to be a little bit darker as well! Not to mention it has a distinct smell...
[shrugs] You haven't done any cooking have you. [smiles knowingly]