You got shot in the head; you can come back from that a totally different person (frontal lobe damage), in addition to memory loss. It wiped the slate clean.
Now, having a past come back at you, one which you don't have any idea about, provides cool storytelling ideas, and many questions for existential things like, 'Am I me? Am I someone else? Am I that person if I have no memory of it?'
I agree here it is a typical story telling device that through an event (in this case a bullet to the brain followed by living room medicine) the main character has a past they do not remember or were running from and want to forget which placed them in the middle of the life changing event. With Geralt it was the trip to the future facting down Alvin and the huntsman, in DAO i always RPed it as a side effect of the joining in FONV it is the tramatic head injury. In FONV the story tellers provided this event as a way to ensure that for RP purposes the main character can be flexible even in the case of a character half or three quarters done with the story/game. In the long run though as all of my FO3 or FONV characters are female it really does not matter one way or the other to me who they were at one time, to me all that matters is who they are now and that that choice is mine to make, and that the PC/myself/story remain flexible enough for any changes to be plausable.
Asai