I think 40 with respect to Biblical generations/genealogy also can be used to denote, in general, an 'unknown number'. I am a (non-professional) genealogist (for almost 40 years). Based on the standard number of years used for a generation; namely, 20, if you do the math, it just doesn't add up correctly with respect to historical time. In short, there are a considerable number of generations missing (typically "filled in" by assigning people inordinately long lifespans).