Preston Garvey was leading the MinuteMen...by default that made him the General. (Though he never claimed the rank in the story.)
He promoted us to General.
At that time he, in his own demented mind, promoted himself as well.
What is one step up from leading the army?
Leading the entire Country.
So, "Preston Garvey" promotes himself to "President Garvey".
Pres Garvey!
It's just what I tell myself so that I don't plot his death every time he sends me to a settlement in distress.