yes you are missing something
6 2000mAh AA batteries
can give you
12000mAh at 1.2V or
6000mAh at 2.4V or
4000 mAh at 3.6V or
2000 mAh at 7.2V
maybe it would help if you work it out in w/h
12000mAh at 1.2V or =14.4w/h(l think l have this right)
6000mAh at 2.4V or =14.4w/h
4000 mAh at 3.6V or =14.4w/h
2000 mAh at 7.2V =14.4w/h