Beyer Help, Please

[NoLuvForPletch]

In my neverending quest to understand The Beyer figure, I have a new question.

A filly runs on May 2nd, a 6f turf sprint, and earns a 78 BSF. I bought the figure on DRF a day or 2 after the race. This filly runs back on May 22, and the 78 BSF is in her PP's in the DRF. The race sets up horribly for her and she is given no chance in the race by the jock's poor tactics, and is subsequently given a much lower figure, and deservedly so. She is entered again on June 19, but in her PP's for the May 2 race is an 80 BSF.

Can anyone explain this to me? Isn't that like a 4-5 length difference?