sterretje answered all your questions. I can add to that, that as far as I know the chip die is the same. It is just the plastic package, DIP-28 or TQFP 32, that is different.
The DIP-28 does not have enough pins for A6 and A7, so they don't have pins to the outside world. However they are still inside (in the chip die), it is even possible to read noise from them.