Simple use-case. A merchant requested a payment through PayPal in Euros. I have connected a card which accepts Euros. I have set my primary currency in PayPal to Euros. When the payment reaches PayPal, they charge my account in Pounds. Not only that, they used their own currency exchange resulting in further additional costs. After exchanging 4 emails with the customer support, they let me know that each card has another setting regarding which currency it is being charged with that you (the consumer) cannot see or change unless you send them a message (in which they reply after 24 hours at best). They also refused reimbursement even though it was clearly their fault.
To generalise, the issue of which currency to charge should be left to the consumer, not PayPal. In the above case, they overcharged me with 8%. And all of that because of hidden settings (that do not appear in your profile btw).