I don't see your trigger so I can't be totally sure of the reason this is happening.
You can check the prices of the selections at the moment of placing the bets. To do this, add the following action to the betting action of your trigger:
write to action log
Text: [r_1_back_price], [r_2_back_price], [r_3_back_price]
Then, after a bet is placed, simply check the prices of the three favourites in the Program Log, and you will have your answer.