I'm going to talk about the past again, big surprise. But, there have been successful orgs that draw so much talent they can have several aged teams in one age group. In the early to mid-90's, I played for an org. that had many teams, based on what you planned to do with your future softball career. If you just played HS, and didn't intend to play in college, you knew that by about your sophomore year.
Team A: all the "serious" players, higher dues, bigger commitment, more travel.
Team B: equally talented girls, that knew they wanted to end their careers after HS, but had HS coaches that made them play TB, less travel, less dues, kids could work during the summer but fulfill their obligation to their HS coaches.
There was a "serious" and not so "serious" team at 18u and 16u. Just b/c you didn't play in college didn't make you a bad player. Some kids don't play in college b/c their parents don't want them to, they are choosing a tough major, they can afford to pay for college so why bother. I think in the end it gives kids that like to play fastpitch the opportunity to continue, especially if they are used to playing pretty seriously up until that point. The kids have the reasoning of, "that's what I do in summer...play softball".
At younger ages, same thing, could be based on seriousness. Do they have parents willing to work with them and take them to different coaches weekly or not.