SID’s instead of usernames..

Can anyone tell me why I see SID’s (S-1-S-91283749823-12908391082374) instead of a username (joeblow) in many different area’s, but specifically in security policy
key:logon locally – administrators;S-1-S-981391827-1298391872;maryjoe;joemary;
etc..
Microsoft seems to be oblivious to this issue, although I’ve seen it on several occasions. And the second question is, does it matter?