Facebook friends mystery

What puzzled me in the past week was the varying number of my Facebook friends. I would get that it decreases from time to time, since someone who you met years ago no longer wants to share their life with you and removes you from freinds. I do that to other people and I assume that other people do that to me(even though for some reason there is no notification for that). But that is not the reason why I was confused. What surprised me, was that the number of my friends grew without me approving any friend requests. How do you explain that one, huh?