Its 99% probably caused by improper linking which some affiliates mistakenly link to tour links with their nats code WITHOUT passing through the /track/ NATS link code provided.
For example:
Visitor goes to Affiliate A's site whereas he clicks on a NATS link and sets a cookie/session for that user.
Visitor then goes to Affiliate B's site but Affiliate B is linking to
http://www.scoreland.com/?nats=XXXXX instead of
http://join.scoreland.com/track/XXXXX/ (proper link code that sets cookie/session).
The user then signs up with the referring URL of Affiliate B, but credit was given to proper linkcode that set the cookie/session from Affiliate A.
/class dismissed.