>>I imagine that it's because in the binary system, 11 only equals decimal 3 because 2^0 + 2^1 = 3. Otherwise, you'd have 2^1 + 2^2 = 6. If the 1st position isn't '0', then how would you represent decimal '1' in the binary system?
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>While internally it is like that, if, in .NET for example, some items are one based as oppose to zero based, I assume that, on the other end, they could have made all of them available either as zero based or one based but not a mix of both.

I assume, like you, that they could have, but since internally, it starts at 0, it probably just seemed natural to do it the same way externally; at least, at first. Later somebody probably thought it seemed odd to start at 0, so changed it, and since then, everybody creating a portion of a language does it whichever way feels most natural to hiim/herself. You have to wonder if anybody is enforcing any standards over an entire project.