you don't know who gave other people reps...
a few days ago I saw also a usernames in my rep sheet but I'm posting from university now and I can't see the names here and I probably can't see them at home either now...maybe it was some bug testing or they're preparing the system for the user to be able to see who gave him rep.
anyway If they don't make the one who gives your rep visible then just identify yourself when you give rep so people know it's you...I'm sure when they rep you back they'll do the same.
"Hey, you noticed something? We somehow kept this topic (green dots, i case someone forgot) until here... I guess we all can give each other good reps for this"
yes we can...I have some rep spreading to do before I can rep you again...If I remeber correctly I reped you twice...