This is taken from the PayPal Express integration documentation:
As a convenience, PayPal fills out the billing address fields with the buyer’s shipping address.
This is a limitation of PayPal and cannot be fixed. Our website is only able to send a single address to PayPal. For normal PayPal payments (as opposed to credit card payments) this address has to be the shipping address, as PayPal already has the billing address. There is no way for us to know ahead of time whether the customer has a PayPal account, so we can't send the billing address, such as when the customer is not paying with their PayPal account, but a credit card instead.