Remarkety is actually more accurate in tracking generated revenue than Google Analytics is.
Google Analytics will only count a conversion when a user clicks a link and makes a purchase all within one user session.
Remarkety tracks purchases via email address rather than cookies, user sessions, etc. This allows conversions to be counted over a larger window of time and across different devices. By tracking via email address, Remarkety helps you to truly close the loop on a customer conversion.
Also, Remarkety's conversion attribution model is set to 4 days since email is sent. If a conversion has been made more than 4 days after the email is sent, Remarkety will not track this order as a conversion while GA did track it as a conversion. You can contact firstname.lastname@example.org anytime if you like to have your conversion attribution model modified.
To read more on how Remarkety tracks conversions, see here: How does Remarkety track conversions.