AFAIR, the over has a lower winning percentage compared to the under. I don’t have the exact data to back it up right now, but that’s what I’ve consistently seen mentioned in betting discussions and analysis.
This is not true. Overs generally occur more than unders. Now, this is not in all cases because in some leagues, if we're using football, they score more goals than some. In Basketball, I hardly bet under in the NBA in terms of points, but I can do that in the WMBA or mature league or even other leagues in South America and Europe. Bottom line is, it depends on the game.
If we focus on football, Overs occur more than Unders. The most popular types of overs are corners, bookings and goals, and it is relatively better to bet on overs than users.
For goals, let's take the ongoing Club World Cup for example, you'll see that out of the seven games that have been played in the round of 16, only 2 games did not produce over 1.5 goals. Over 1.5 and over 2.5 are the most popular overs for goals I football, and more items are played over 2.5 than under, and that is why the over 2.5 odds are usually smaller than the under.