Let say if real Madrid is playing against Manchester City, selecting over 2.5 will be a big possible winning bets since we are sure that all the entire time of the match Both clubs will scores and the total score will definitely for up to that amount in odds so winning such a game is easier.
But in such match, did you noticed that the odd for over 2.5 which use to be very high in other matches will be very small in this kind of a match? You can see it around 1.2 odd or probably less. This will encourage people to go for higher one like over 4.5 which could be of possible loss for the person. I have noticed one thing about bookies, they give unfavorable odds.