I have 3 matches like that with public trees that range over 10000 people big. I sent a meaasge to the first 2 I found just in case it was some sort of Ancestry bug that caused it but I got no response from either person.

I tossed the trees into the trash with the knowledge that the majority of the uber trees that I've checked out were 3/4 BS with their direct links to Adam & Eve or every Royal ever born or some equally unlikely "ancestor".