I don't think it is "most" (but that's why you check with the schools before you apply)... many schools are aware of many prehealth committee letters' timeliness often being late and adjust at their discretion. Your app might be held until the letters come in for a interview invitation. Others might hold a decision to offer until they get your letters.
Many admissions professionals assure the prehealth advisors every summer that it is not a negative if they send letters in August or September. Take it at face value.