It’s one of the reasons that regulation would help a lot here. One simple example is chelation. Your body absorbs magnesium when it’s chelated at a rate many multiple times higher than what you might typically get from an OTC multivitamin.

So a multivitamin might contain some magnesium, but what it doesn’t tell you on the label is that the form of magnesium they give you is not going to be absorbed at all and just pass right through your body. Most people need some other formulation of magnesium to actually be absorbed. I would go so far as to say that a fair amount of what goes into a lot of multivitamins on the label is borderline fraud because of stuff like this