I found word of Google Product Ads which behave in this strange way:
How much does it cost? You specify the commission rate for conversions that take place via clicks on Google product ads. Minimum pricing is your standard publisher rate, plus the network fee. To maximize your competitiveness among advertisers participating in product ads, we recommend a higher commission with the Google product ads relationship.
I bet this has been reported here before, but I can't find a matching thread.