Converting DC battery power to AC wall power loses roughly 15% in the inverter. Manufacturers quote the battery's rated Wh, not what reaches your appliance.
What shortens the runtime?
Battery age (−20% per ~2 years), cold weather, and the display/inverter idle draw on small loads. Treat 21 hours as a good-conditions estimate; plan on ~17 for an older unit.
What if I run other things at the same time?
Runtime divides across loads: add a 12 W router and the same battery lasts about 11 hours instead.