Every year the center manager allocates specific lanes to a specific league. Some leagues do not start at the same time nor do they end at the same times (4 person vs 5 person leagues). Plus some take off certain days of the year while other bowl. They also want to insure they have enough lanes away from the league for open play after 1 or 2 or the leagues are finished for the night. Also, some centers may put down a easier shot for mixed leagues vs another competive league. This is just plain good management and an extremely common practice.