Great question. Sometimes generous merchant's are paying you extra for the lifetime value of the customer. For instance some hosting companies pay 65 per sale even on 9.95 hosting accounts, knowing that most of their customers stay with them long enough to justify that commission.
Also sometimes the customer is locked into auto-ship. I know one vitamin company that was like that. The initial sale was 35 I think and they paid out $6o commission, because customers would be making auto-repeat orders.
Interesting to note - just went to the site to double check my numbers.
They killed their affiliate program and no longer offer one. 
2 things to keep in mind:
Merchants will never pay you more than they make. If they pay over 100% commission they are making extra money somewhere.
Sometimes if an offer SOUNDS too good to be true - it really is.
(Some programs could be scams.)