It comes down to breeding. Just as some dogs have overly long noses or other departures from the breed standard, some others have insufficient jowls (and occassionally, excessive jowls).
Breeding isn't an exact science - sometimes you don't get what you're aiming for. And of course, some people are just breeding indiscriminately (usually their pets, without regard for whether they're accurate enough representatives of the breed to be used for breeding, or whether the pair are a good match, etc). In the latter case, it doesn't take long for the distinctive look of a boxer to start to disappear, usually beginning with the head (jowls included).
If you are interested in what is correct (as well as why there are differences), you may find this page interesting - particularly the "Munich silhouette" which gives a good indication of how much jowl a boxer should have:
The Head