You often hear people say things like ”do you mind if I sit down?” or ”do you mind if I eat the rest of your cake?”. And the answer usually is ”certainly, go ahead” or ”yes, I have had all I want”.
This seems very strange to me. Is it something I don't get? If someone asks if I mind, and I don't, wouldn't the natural answer be along the lines of ”no, I don't mind” instead of ”yes, bla bla bla”? Yes, sounds like you do mind.