Was my mom a slut? DNA testing results inside (aka I'm confused)

See the attached for the results of DNA testing of me, my two brothers and my mother. Our father was not included. I'm terribly confused as to how siblings 1 & 3 are absolutely related, and siblings 2 & 3 are related, but how it's only "possible" that siblings 1 & 2 are related.



Can someone give me some explanation? I'm a lawyer, not a genetic scientist.