The math does seem to check out. Assuming it was bet like a 6-1 shot in the P5, the 6-1 shot would have taken around 12% of the pool. The winning favorite in the 9th paid $6.50 to win, representing about 25% of the win pool. When the P5 money on the scratched horse is shifted to the favorite, the P5 money originally on the favorite goes from collecting the full net pool to collecting around 25/37ths of it, reducing the payout from $1106 to around $747.