That's the thing, Chewett, coins do not keep their ITCs(as far as I know) when a new one is added on to it.
So, if you have 15 coins, and coin #15's ITC is 12345. When coin #16 is added, #15 loses it's ITC, and then gets "covered" up with #16s. When you trade #16. #15s is different. Again, this is just my understanding. Not tested.