Were there people on the list? If not this isn't that crazy - it does happen from time to time.

My JFK - RDU flight last night went out with 1F empty and the 6am RDU - LGA flight yesterday morning would have gone out with 2 empty in F but RDU - EWR was delayed and 2 of us (including me) who had been upgraded to F on the EWR flight were moved over to the LGA flight.

A mid-week, mid-morning flight isn't exactly peak time so it's not completely crazy to think that the medallion count is light.