Why are we still using the substraction method for the Holocaust?

Everyone knows that many Jews went into secret exile, having to flee to various surrounding countries, the Americas and of course their settler colony called Israel. They also changed their names and obscured their ethnicity. Still today Jewish people are finding out that many relatives are still either alive or resting in graves or various countries around the world. So why aren't we using these examples as a way to correct the record once and for all? There are many assumptions that have to be made in order to come up with this number. Why aren't we being more accurate?