Consider the response of both to heat. There's a good explanation for why polyunsaturated fats are more healthy when raw, but also good reasons to think that they're less healthy when heated to cooking temperature. But studies, and people, tend to ignore the distinction as if there's no chemical changes in response to 400F heat.
Add to this the fact that, for example, "sunflower oil" can vary wildly in it's actual lipid structure because of genetic variation, things get even more complicated.
Sure, there's manipulation in nutrition science, but it's not that different from other biomedical sciences. A lot of the confusion is that, but a lot of it is just random sampling variation, and problems people have with interpreting meta-analyses rather than individual studies; but some of it too is the actual inherent complexity of the topic.