Logic behind CI naming conventions...

[eluser]Aken[/eluser]
Class names should always be ucfirst-style; no other capital letter should exist except the first letter.

Filenames should either match the class name, or be all lowercase. Sometimes lowercase can make your life easier depending on the case sensitivity on your server.

Remember that this is not just specifically related to filenames - the Loader class uses he same guides when you use $this->load->library() or similar. That's what allows users to load a library in all lowercase.

It's not the best solution, and I doubt you'll find a thought-out explanation for it from an original developer (it's been like this for a long time).