Are you sure that your username and summoner name aren't the same?
# 2 1/18/2013 2:00:00 AM
# 1 1/18/2013 2:00:00 AMI guarantee you this problem can be reproduced just by clicking the Login button and entering your e-mail in the "Forgot Username" field. Check your e-mail and it will return your Summoner Name instead of your Username.
EDIT: Also, the reason exact reproduction steps are not required is because the query for your username is server-side and thus has nothing to do with what I'm running.