mAh and Wh ratings not matching with voltage of device
This answer was originally by @winny in comments. He isn't interested in making it an answer so I'm doing so as I feel it is a valid answer and should be posted as such.

The internal battery is most likely Lithium-Ion, producing 3.7 volts (nominal), which is fed into a boost-regulator to produce 5 volts. The 20 Ah rating is on the battery, not the 5 volt output. Thus, 20Ah * 3.7V = 74Wh.