When it is said that Boxer A has Boxer B's number, it implies that no matter how many times they meet, the outcome of Boxer A beating Boxer B will always be the same and for that to be true, they must have met in the ring more than once. Why are we seeing examples here where there was only one meeting between boxers (i.e. De La Hoya vs. Sturm) or multiple meetings where each fighter holds at least one win over the other (i.e. Holyfield vs. Ruiz)? Forrest vs. Mosely is a great example because they have met more than once and Mosely has not been able to figure Forrest out since their amateur days. However, Forrest hasn't been the same since getting blown out by Mayorga. If you're not impressed with certain boxers, the same can be said about Mayweather vs. Castillo, Mosely vs. Vargas, Arce vs. Hussein and Peter vs. Toney (all were 'more-than-once' encounters and the victors' have all been considered less than worthy, but it all depends on who you ask).

Edit: Forgot to add the perfect example, Maskaev vs. Rahman.