Hypothetically, you find out your dad changed his last name because he hated his father. Let’s say he changed it to the last name of a fictional character he really liked, then went completely no-contact with his family decades ago, before you were even born. Then, 20-something years later, you discover your grandmother is still alive, and your last name isn’t actually your dad’s birth last name.
What would you do?
Phrasing aside, it’s still asking strangers to speculate on the reasoning of one person. That is definitionally a subjective answer. Even if someone magically gleams the reasoning, it cannot be confirmed without their dad confirming it.