In a two-horse race, the chances of each horse winning will only be 50-50 if they are equal in all respects. As soon as one horse has an advantage over the other, then it is more likely that that horse will win.
How likely it is will depend upon how much better one horse is over the other.
Lastly, the quoted figure of favourites winning 33 percent of races is an AVERAGE over many races with varying numbers participating in each. Obviously, the bigger the field, the greater the chance of one of the others beating the favourite.
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Here is another way of looking at it.
Consider three races each with 5 horses.
Your statistics would predict that the favourite would be beaten in two of those.
The favourite has raced against 12 individuals and has been beaten by just two of them.
If those had been raced as individual two-horse races, then the favourite would have won 10 and lost 2.