MS17-010 Unexpected Behavior

Hoping someone can help me understand an issue I ran into today.

Two boxes, both vulnerable to MS17-010, both allow SMB sessions using username ‘’, password ‘’

On box 1 (windows xp), my manual ms17-010 exploit ( only works using a username of ‘’ in the .py file

On box 2 (windows 7), the same exploit only works when I edit the .py file to use username = ‘//’

Is this a difference between Windows versions?
In both cases – to my understanding – the exploit is using anonymous/guest access, so why the different usernames?