The power bank has two regular standards of capacity, one is the battery capacity, one is the rated capacity.  The  20,000mah power bank, being labeled 10000mah/3.7v core capacity be converted to rated capacity 12400mah/5v.

Why exist this difference?

The most important reason is the conversion rate. Because most of the power bank’s battery voltage is 3.7V, and power bank needs to increase the voltage to 5V when charging the mobile phone, which making the power bank’s conversion efficiency only 3.75/5=74%, Also include generated heat when battery voltage boosting, and various of wire consumption, Electricity, and self-consumption. these points add up to account for 30%-40% of the power of the mobile power source. So the 20,000mAh power bank that can really be used by us is about 12400mAh.

Also, The conversion rate of the entire mobile power industry is basically in the range of 60%-70%. Please kindly noted.