It certainly hasn't happened to somebody who knows how the shoe should look. Flyknit racer fakes look nothing even close to the real deal. It might be true for other shoes, but all my Nike purchased racers look slightly different and have inconsistent fit to them... But the fakes look terrible. If I were to use a scale, see they weigh differently, and call up Nike telling me that they sold me fakes, how well do you think that would go?
By your logic, we should come up with the weights for the shoes, and have people reference the chart to see if they are fake or not? Each size and style would need these weights, and we would need large sample sizes of shoes. Instead, we can look at the shoes and easily tell if they are fake or not, when looking at racers/trainers.