This is a terribly simplified explanation, but here goes...
Series did used to run from September to about May, but with reruns and other things thrown in to keep "your" show on during its' time slot for the whole year. Television has evolved though, due mainly to the way we watch TV. Back in the day, you watched a rerun of your network show because there weren't too many other choices. That isn't the case today... and from a business standpoint most reruns don't bring in the advertising dollars.
It is fairly common now to break a series into mini-seasons. You hear more often "spring season" "winter finale" ... etc. I know Heroes was one of the first to follow the format.