Heres a story that happened on my flight from STL - MSP on WN yesterday.

Announced an oversell offered $100 plus cost. Original flight arrived 1230p. New time arrival would be a flight from STL-DEN and then DEN-MSP would arrive 430p. I declined the voucher. Another person did it and he got $300+cost. Gate agent gave a different amount then once volunteered since I asked the guy how much he got. He got on an earlier flight to Denver etc.

However there was still one empty seat on my flight from STL-MSP. How come he got the oversell anyway.