In addition to the great many people who breed just because they've got two dogs - which has already been mentioned, even amongst those breeding to the standard, you've got to contend with individual interpretation of the standard. After all, it is simply a written description, and there is quite some variance in people's interpretation of that description and opinion of what makes a "good" boxer.
Just a quick look at the American vs European boxer can illustrate this. The standards being are bred to are almost identical, but in many cases, the results ar not. A typical American head, for example, is longer and lighter than its European counterpart - yet both meet the description of the standard which, inter alia, says the length of the muzzle is a third of the total.
All that being said, if you look at the very *best* that are bred across all countries then you'll find far less variation in type. In short, we're all aiming for the same result but its hardly an exact science (and there'd be nothing left to aim for if it were).