Due to technical reason the user names visible in the Quotas window are the names used when the user accounts were originally created. Names of newly created user accounts are always shown as they are, however once you rename an account, the previous original version of the user name is shown for that user.
There is no workaround for this problem at this moment. However, this should not be an issue as long as you know what was the user name of a particular Windows account when it was originally created.