Why is the outlet ground voltage different from grounded Macbook voltage
Your body has the bigger surface area (higher capacitance) and therefore picks up a larger extraneous signal compared with the MacBook case. 

The MacBook case has a smaller capacitance to ground/environment and as such picks up a smaller signal. In fact, when it is plugged in to the charger, it will have more capacitance to ground than to the extraneous signals from the "environment". However, that capacitance to ground might only be about the same order as your body has to the "environment".

You cannot conclude that the Mac case is grounded. It is highly likely that there is no galvanic connection to ground - there will be capacitance to ground through the charger isolation transformer but this is not a galvanic connection and, as such, cannot be at all relied upon as a "hard" ground.

On this basis, you could expect a reduction in body signal when touching the Mac but there's no reason to believe it will take 53 mV all the way down to 5 mV. In fact by touching the Mac case you will cause the Mac case to rise in voltage. 

Did you forget to  measure this?