In the NL Central--and other divisions--not all of the teams in it are actually trying to win, and in most years, therefore, it has only four or five teams that are actually worth worrying about (and usually fewer). For example, last season only the Cardinals, Astros, and Reds were in anything resembling serious contention.
So this argument is rather misleading. Of course, it’s still a bit unfair this year, because, for example, in the AL West only three teams have any chance to win, but in the NL Central there appear to be four teams with some chance. But that varies from year to year, and ends up having little to do with how many teams are actually in a given division. Even in the days of 7-team divisions, there were seasons in which a whole division had only two contenders.