That is because NYCOMCO, in their ultimate wisdom, has the same frequency (856.1125) running on both Schunnemunk and Illinois. That is ok if controllers are set correclty...now for the BUT....they don't have the controllers set to NOT send to that frequency if it is being busy on the other site.
So, even if you are on your talkgroup on Illinois, if you happen to be assigned to 856.1125 and that is the next available channel on Schunnemunk, once someone keys up...you guessed it...interference.