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:

  PHP:
  $myString = sprintf('Total: %.2f', $myFloat); 

  ObjC:
  NSString *myString = [NSString stringWithFormat:@"Total: %.2f", myFloat]; 

Maybe this is the reason these characters don't score too highly in overall frequency? I have no answer for the high frequency of '/'.