What does your second, grounded, piece of foil bring to the party please?
Capacitance is being measured between the Receive pin and ground. Your body is coupled capacitively to ground. The extra sheet of metal connected to ground increases the capacitive coupling of your body to ground, increasing the reading and therefore sensitivity. Also, if the Arduino is not actually grounded (which is almost certainly the case), then you are really measuring just the capacitance from the Receive pin to the Arduino common.