Yeah, the "+" is doing some heavy lifting there :-) 3x is what you can basically just fall into after military retirement without trying very hard, with a bad network, and with poor luck.

Another factor is that your expenses can be quite low while you're in, even with a family. Put an officer's salary on top of that—which isn't amazing, but isn't terrible either—and you can save a fair bit of cash even before you even start the contractor + retirement double-dipping.

Though there are real costs—the schools are generally very good, but you may have to move every couple years. The bureaucracy is hellish (though, for all their dysfunction, they've got a lot of shit figured out way better than your average bigco). You'll probably serve under and with some real assholes at some point. And there's ever the temptation to get out early and start taking those sweet contractor dollars before you secure the government retirement check. Plus, you know, you may have to go kill people or get killed or what have you.

[EDIT] Oh, another downside: it can be really hard for your spouse to have a career, due to all the moving and often living in places without much economic opportunity.