Check this out:
http://www.ingayporn.com/whos-checking-mancheck.html
Seems that Mancheck sneaked a link on every avs entrance script to their own site, and of course it doesn't include you ref code.
They are aware of this issue.
I believe someone (Gleem?) posted about how misspelled words will draw clicks? I wonder how many free sales they get from this.
So with Mancheck, YOUR traffic, can join YOUR SITE and you wont get credited for it.