Facebook friends question

I changed my settings to private recently as I don’t like my life to be on display etc… I noticed that 2 of my friends (we are all friends), they aren’t showing as friends with each other on Facebook - so they don’t appear as mutual friends if that makes sense. I haven’t restricted anyone, so I’m a little confused if this is something that I’ve done or if this is something they’ve done to their settings. Honestly, I’m so confused by Facebook settings, I’m better of getting rid with how complicated it’s getting. Before I go on a rant - does anyone know why this is? I don’t think it’s me?