I had always thought anything pure was 100% and nothing added. Well not anymore we buy pure cod liver oil thinking it was 100% recently noticed despite being pure on the box it contained 2% tuna oil.
Had a look around the supermarket many products marked pure have added stuff.
Tuna oil contains good things that Cod liver oil hasn't got, and vice versa, so a mixture of the two may be an advantage. Too much Cod liver oil can result in nasty side effects. It shouldn't be taken as a substitute for fish oils.