How to compare mAh to Watt hours?
Lithium ion cells have a nominal voltage of 3.7 volts, so the pack has a total (nominal) voltage of 11.1 volts. Power equals voltage time current. 57 Wh/11.1 gives a current capacity of 5.13 Ah, or 5130 mAh.

A powerbank normally has a single cell voltage, so a 26500 mAh bank can supply 26.5 x 3.7 = 98 Wh.

Choice of units is largely a matter of tradition. For batteries, since any "normal" load will run at battery voltage, rather than some arbitrary voltage, power and current are exactly equivalent. Therefore, total charge and energy are likewise equivalent. However, it's easier to measure current than it is power, since after all, power requires measuring both current AND voltage. So, for a known, more-or-less-constant voltage such as a battery it's just easier to measure only the current and be done with it.