I find most will just copy-cat from the conventions used in examples. Makes sense. Just going with what another programmer of that language/script is doing since it seems to work.

For example, I was originally taught to use i, j and k consecutively in my for loops. What sense does this make in practice? I have no idea, I just use them instead of say... a, b and c for example. Though this doesn't become a readability issue unless I'm trying to use the darn thing globally or something weird like that.