Top leagues offer a variety of different length contracts, from a few weeks, to half a season, to a full season. Teams do not stay the same over the course of the summer; some players arrive late, some players get injured either in college or during the summer, some show up with more innings from college than expected, some leave for other reasons. At the start of the summer, teams will add players (especially pitchers) to make up for this, and by the end of the season, many summer teams are scrabbling for whatever players they can get, even in NWL. My son's contracts were coach-organized in the fall in the past 2 years, but they changed as circumstances changed.