Our Terms and Conditions of Service do not allow our service to be used to transfer money from one person to another because that would be a regulatory nightmare and fraud magnet. So if you make a payment from a PayPal account with an email address that does not match your contact address, our system automatically assumes that the deposit came from someone else and may warn you that it's nonrefundable.
In practice, this only matters if you close your membership with a remaining balance from such a payment. Even then, it is rarely an issue.
We can always attempt to refund the money to the PayPal account it came from, whether that was you or not. In rare cases, usually when the sending PayPal account has since been closed, that might not work.
If you are John Example, your contact email address is john.example@gmail.com, and the payment came from john@example.com, we'll probably be able to do the mental heavy lifting required to connect those dots and get the money back to you. But we can't guarantee that; it's a little subjective.
If the sending PayPal account no longer exists and we can't positively correlate the original payment to you, that can result in us being unable to issue a refund. (Hence "nonrefundable.") Even in such cases, we don't just keep the money; donations to charity or transfers to other members are still possible.