When I had found values for the 4 answers of a set, and had solved the hint, I decided that the relationship was the difference between the values of each pair. This enabled me to complete the puzzle. But when I saw the official version of the relationship clue it was different. Of course if A+B=C+ D (1) then A-C=D-B (2); but the pairings would then be different.
Does this matter? If A,B and C are known, then using (1) there are
in principle 3 possible values of D; but using (2) there are 6 values, as D could be greater or lesser than B. But a little more thought shows that while there are 6 answers, there are in fact only 3 distinct values and they are the same as those obtained using (1) So as far as the puzzle is concerned the relationship can be expressed in either way.