Your payment processor can see the referers to their page, where your user actually signs up and pays.
Since the user then goes through the payment processor to get to your page, the payment processor page becomes the referer to you.
So, you see the actual referer to your page and the payment processor sees the referer to their page and google is showing you the only information it can.
SSo, the question becomes how does the user get to the payment processor. You would have to code something in there so that you could track which users went to the payment processor page and then actually completed the sign up to get to you and then you could get the information you wanted.
Hope that helps

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