Salmon is an oily fish and haddock isn't, the salmon will get a higher free food allowance for total fat, but the fat in the salmon is healthy par the fat in the haddock if that makes sense. Also the salmon is higher on protein than the haddock, which will also bump up the free food allowance.
This is one of the reasons you should never assume something is low syn just by calories alone, as the syn value when there is a free food is worked out with a very complex formula, which not only looks at nutritional information but also the type of free food too.
Hope that helps