In standup it was stated that this was only coming from amazon mobile and iPad amazon users.
As part of our reporting on the Big English campaign, I've been looking into the referrer column in contribution_tracking more. We are already excluding referrer = 'googleweblight.com/' from our analysis (due to T152602).
However there appear to be a large number of entries with NULL referrer. These are spread across banners and email, and particularly common on mobile and iPad. For PayPal and Amazon they seem to convert well, but for credit card they rarely result in a donation which is dragging our rates down.
Do you have any thoughts on what could be causing these, and why it would have an effect on conversion? Would it be something to do with the fraud filter?