Using a published Watt-hour power curve for AA batteries, I get a theoretical battery life that's way lower than in practice
Brian Drummond (see comments) is correct.  For each battery that you have:

$$Power = V_{batt}I_{batt}$$



If you put the batteries in parallel, your voltage doesn't change, but your potential current changes:

$$Power = V_{batt}(8I_{batt})$$



If you put the batteries in series, your potential current doesn't change, but your voltage does:

$$Power = (8V_{batt})I_{batt}$$



Either way, you get 8x the Power with 8 batteries:

$$Power = 8V_{batt}I_{batt}$$



Add in the time factor and you have Amp-hours or Watt-hours.