Yes you are right, they can fit larger cards but then you run the risk of nipping a corner when you try to fit the larger card in there, the premium sleeves are made thicker giving the card more protection and less room for thicker cards.
My George Romero card came in a standard sleeve, it looked bent and creased until I took it out of the sleeve as they were not designed for larger cards.
I only buy sleeves to use in mailing cards to others so I dont want to spend top dollars on products that arnt for me as all my cards go straight into One Touch cases.