That's the point. If they see atrophy in people who drink 14-20 units per week, that leaves room to wonder whether there's a significant difference between peop

That's the point. If they see atrophy in people who drink 14-20 units per week, that leaves room to wonder whether there's a significant difference between people within that group depending on whether they drink 2-3 units every day or drink 4-7 units three days a week with four rest days. You could then go further and see if it is different to have four consecutive rest days and three consecutive drinking days versus having each drinking day spaced with at least one rest day. Etc. The point is that lumping everyone who has 14-20 drinks in a week into one category makes for a large variation in behaviors and possible interactions.