Uh... because the person who was in second had one person already in front of them?
So when you pass that person they become third (they have two people infront of them), and since you are in front of the person in third, that makes you second.
Unless the person who was second was tied for first or that when you overtook you passed two people instead of just one.
Or think of it like game rankings. George in first place with 100 frags, harry in second place with 50 frags you somewhere below harry with 45 frags. You drop a setta grenades/launch a redeemer into a room with a setta n00bs shooting one another up and rack up 6 frags. That means that you now have 51 frags, you just passed the man in second place, you would now be in... second place.
The whole point is, it doh really matter in the end, what you need to study is not what palce you in now... but where them n00bs respawned and if you could get off another clean shot like that.. and look out for frekking george, that man doh dead. Is like he sitting on a big keg of health and a shield belt that keep flipping spawning in when yuh now get him damage good. Boy that bastard camping. Look lewwe go take his bitc#a$$ out.