Can someone explain added time to me?

I don't watch or follow soccer but I enjoy career mode very much.

I notice they add say 3min but yet they'll let the clock run 5, 6 or ore minutes past? Especially if the other team is attacking. For instance they had 3 corner kicks in a row even though the first one took place well past the added time. Why is that? In any other sport clock runs out it runs out, it doesn't matter what position a team is in.

Just curious why they say 3min added time but will quite often let it run well past that mark.