Long, long ago, member records were stored as files using the login name, so looking up a member record using the login name was easy.
Then came the switch to member numbers, which meant an index of names and numbers needed to be kept - memberslist.cgi
That could be slow, though, so in 6.0, Ted started storing the member number with the login name so that looking things up in memberslist could be skipped.
That's why they're both stored.
