I think the popularity of “www” is bogus (and likewise some of the others). When you look at the popularity of the first label of the domain name across all hosts on the internet then domain names that are named functionally (like www, mail, ns, ftp, smtp, imap) will be popular. However the actual implementing host (even if there is only one) will probably have a real and different hostname. In other words, the domain name label (like www) is some kind of alias.
This is good practice because it allows you to shuffle the services around a set of implementing hosts without breaking clients.