You can keep comparing them to Webb, but ultimately Webb is constrained by district boundaries, accept every student who shows enrolls, regardless of mental, physical, or behavioral issues, and are playing schools with between 350 and 600 boys to choose from (in Class 4).
Valle meanwhile draws from an area with ~20,000 people, can reject mental and behavior issues, but plays schools who are constrained by district boundaries and usually are rural with towns of less than 1000 people and between 60 and 110 boys to choose from, not taking into account the boys in those rural schools who HAVE to work, have mental or behavior issues, aren't athletes, or don't play for whatever reason. No that isn't Valle's fault, but you can't say Valle or even St. Vincent play under the same constraints as Thayer, Miller, Marionville, Mark Twain, etc.
Private schools have inherent advantages that are magnified when playing against the smallest schools in the state.