The breed standard only applies to conformation judging (i.e., dogs in performance events are not judged on how well they adhere to the standard in any way, only on how well the perform their tasks). I'm surprised, though, that you are seeing shorter Boxers - in most areas they're getting taller! Or, it could be that you're used to seeing taller dogs and so the dogs that are within standard height look smaller. (My oldest girl is 1/2" above the low end of the standard, but everyone seems to think she's tiny.)