Boston keeps a table of qualifying times that need to be hit. But it's such a popular race, that people train to hit that qualifying time, and they end up with too many runners. The solution is to use a cut-off time to adjust all of the qualifying times. As an example, an 18-34 year old male had a qualifying time of 3 hours exactly for the 2025 Marathon (which qualification ended for last month). The cut-off time required athletes to be 6 minutes and 51 seconds faster than the qualifying time for their age group and gender. So qualification was actually 2:53:09, but no one knew that when they were running. (Next year they've adjusted the times all down 5 minutes to compensate, though they'll likely have around a 2 minute cut off, since the times have been getting faster.)

https://www.baa.org/races/boston-marathon/qualify