Why do people say "heads up" when really you should duck?

well if you think about it when they say heads up they mean they want to let you know what is going to happen it could be in the near future it might be i want to give you a warning so and so is coming over or someone will be calling or i have to do something tomorrow just wanted to give you a heads up on the situation or something is going on just wanted to give you a heads up on the bus is coming to some it might mean duck your head some think that way