s_pugh:
("all answers and entries are different" means that none of your 21 answers (entries) to the clues will be different; a specific clue may have more than one answer that results in the same prime and grid entry. As I pointed out, 12 down is guaranteed to have several answers that all lead to the correct entry and prime.
The solution to the puzzle is unique - that is there is exactly 1 set of grid entries and primes that satisfy all of the clues, with no duplicates. In most cases the clue ANSWERS are unique but a few, like 12 down, have equally valid answers. If you gather up all of the candidate answers for the 21 clues that generate the unique grid, you will have a set of 26 numbers with no repeats - so all answers are different.