I really don't know for sure. I was trying to delete a zero poster (long boring story) and somehow managed to delete the member. How her posts got attributed to the actual deleted member? Really I do not know this - perhaps a bug in an addon? I honestly can't say.
But a HUGE shoutout to @AndyB who saw this thread and reached out to help me and restore the accidentally deleted member.
This is not the first time
@AndyB has rescued me from a self-created disaster. Andy is one of those all too rare people with a heart of gold and the Xenforo and database expertise to back that up. I cannot speak highly enough about Andy or about his addons (over 300 in his catalogue at a price of $35 for as many of his addons as you want.