Why did his friend add me on Facebook?

I added my guy friend a few weeks ago but I've known him a few months.

I've never met his friend or even heard of him. I think they're childhood friends from his hometown so there's no way we've met before.

The funny thing is when I confronted my friend about who he was, he just apologized and said it was his friend. He didn't say how close they were or anything
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Also me and my friend have never posted on each other's walls so I'm not sure how his friend found me since it's been weeks when the 'new friend added' popped up in his newsfeed.

I'm getting paranoid because I keep my profile on private.
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He claims to have hit the wrong button 😂😂 I think I scared him off
Why did his friend add me on Facebook?
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