Because in big letters it says XX% Bonus, Y times rollover.
Everyone knows that "rollover" refers to the amount of times you must play the deposit plus bonus to keep the bonus free and clear. The amount of money in play is the amount risked whether or not it is a favorite or dog.
What's deceptive at some books is that you have to dig down into the fine print in the rules to read that it is the lesser amount and not the amount risked. While this may be acceptable because they state it in the rules ahead of time, it is misleading since they advertise, "XX% Bonus, Y times rollover" which on its own means something else entirely.