The technical reference for the FDA's definition of bioequivalence is in the article, and here it is again: https://www.fda.gov/media/70115/download

The problem is two-fold - one is that they only demand equivalence with respect to two variables (the peak level, and the total amount absorbed.) The second is that for even these two variables the generic can be as little as 4/5 or as high as 5/4 of the brand name value, and that is just probably too much slop for certain drugs.