Those power figures make no sense, I'm not sure I get it. How could it run for more minutes than the battery size divided by average power draw would imply?
That is be power draw at the outlet [0]. There is probably loss that occurs when drawing from an outlet that does not occur when drawing from the battery.
There is loss to convert AC/DC, likely low 90s efficiency. Having an in-line DC power meter should be a very simple task, measuring the draw power from the outlet is rather pointless.