That's true, but what was actually measured was the mean time to serve the entire request. I think in nearly any usage pattern, this would be the data point that you would be most concerned with. You want users browsing your pages to get their pages and small images very quickly, and if they have to wait a couple seconds for the larger images to finish, or for minutes for large file downloads, that seems to be the kind of behavior a user would expect.
What this scheduling algorithm avoids is the case where several people are downloading large files and other more casual users are having a worse browsing experience because of that.