Ever notice how similar a lot of the "English" alphabet's characters look? The guy who put this together must have really phoned it in that day.
He really could only come up with these:
and then just flipped a few around a bit to get up to 26:
And it isn't just those three, a lot of the original list look eerily similar to each other as well, but I guess we'll count them:
but then he needed to come up with 10 more numbers... well none of those are unique!
So in the end, if you are looking for unique characters that you can use, that will not be confused with any other characters, you're only left with:
well that isn't very many...