They’re not. The 3% is a blended average. The rate varies by the specific card kind, the card brand, the specific vendor in Apples case when they have leverage - or the merchant category code when they don’t have leverage. Even chargeback history matters.

Apple likely has a deal that borderline costs merchant acquirers money and that gift card vendor is probably paying 10%.

Merchants pay different fees depending on their risk category. High risk merchants can pay 15% or more.