As far as I am aware, in the US it is very common to refer to the room that contains the toilet (device for disposing of human waste) as the bathroom. If this is a separate room from the room that contains the bath how do they distinguish between them?
"Where are you?"
"I'm in the bathroom" - doing my private business leave me alone!
"I'm in the bathroom" - washing my hands, it's OK to come and talk to me
In Australia the room that contains the bath is called the bathroom and the room that contains the toilet is called the toilet, unless, confusingly the bath and toilet are actually both in one room, in which case it's called the bathroom. There are now many people in Australia who use the American term "bathroom" in public when they mean "toilet", however, toilet is universally understood and is not considered at all impolite.
Since people found the original title "in the bathroom" vs "in the bathroom" confusing I have changed it.