batteries - mAh and Wh ratings not matching with voltage of device

I was under the impression that
Wh = Ah * V

I have a 20000mAh battery charging device for my phone but am confused about the information provided by it on the side.

It claims to have a 5V output (3.1A max), and a capacity of "20000mAh/74Wh". But following the above equation, 100Wh = 20Ah * 5V. Where does this discrepancy come from?