The $21k/yr guys on the military side of the work force are probably enlisted, not academy or college grads. Those guys (and gals) are more educated than similarly employed civilians, because every last one of them graduated high school (which is not the case among civilians), not to mention that they are probably pursuing courses at the expense of the government.
The office corps are college grad level. In the combat arms, at least, it is a definite pyramid structure, and there are fewer and fewer as you go up in rank. It's not like there are thousands of generals running around skewing the average. As officers advance, they get pruned out of the force pretty aggressively. Now, in non-combat arms branches, things may be different. I am told that there are more officers in support branches.