Wifi Thermostat wiring
The YELLOW or Y terminal at the furncae/ air handler is a dummy terminal for convenience, it is not required to be used,  you could send Yellow directly to the AC unit out doors.
Common will be used since the Common leg of power comes from the transformer which is in the furnace.   The Yellow going to common  likely  if you trace it back it in fact is Common leg of the 24 v power supply.