Unexpected results using ksort on arrays in which the keys are mixed alphanumeric strings.

I have been experimenting using four character alphanumeric keys on an array, and when I generated a random set of keys, and then used ksort to sort the array, I was very surprised to find that if the key contained any non-numeric character, or if it started with zero, the key was sorted as a base 36 number (0- 9, A-Z, as I expected. However if the key only contained numbers, and did not start with zero, it was sorted to the end of the list. Thus: 0000 0009 000A 0999 09A0 ASDF ZZZZ 1000 9999 I presume this is related to last weeks discussions about casting variables, but I cannot understand why 0999 should go to the start of the list, while 1000 goes to the end. Can anyone explain this logically?