I found the Objective-C one strange too. I would have thought '[' and ']' would have been two of the most pressed keys. I only really use '/' for commenting.
I do type a lot more square brackets in ObjC than any other language, but I find that I type a lot more generally due to the increased length of many method names in ObjC (or I would do if autocomplete didn't do it for me).
(Simple) Example:
Maybe this is the reason these characters don't score too highly in overall frequency? I have no answer for the high frequency of '/'.