Used to happen a lot more often than it does. MLB teams used to do that all the time early part of the century. Past few years was also because the AAA team was in Fresno. They put guys they wanted easy access to in AA so they could get here more quickly.
there was a time when AA held the top prospects, and AAA was the major league replacement players IIRC.
I always thought of AAA as where you kept your journeyman, MLB capable guys who could fill in for injuries, etc. And AA as the top level for the high end/high ceiling prospects.
FWIW, Strasburg did get time at AAA before getting called up.
