It has a lot to do with temperatures, possibly some overweight passengers (been bumped from a commuter flight before due to some heavyweights), amount of cargo, etc. They have to be very careful with where the center of gravity is.
I recently didn't make standby on a Northwest DC-9 flight (OMA-MSP) because of weight and balance restrictions... the airplane left with five empty seats.
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Jason Seiple
OMA