When is a bathroom, not a bathroom?

[Brownlow]

For many years, I've been aware that Americans call a lavatory 'the bathroom'; it's always amused me, because it begs the question: What do they call a bathroom (i.e.: a room with a bath in it)?
Adding to the confusion, shower-rooms are now being described as bathrooms.
But, here's the killer...last week, I received details of a house, with 2� bathrooms...what on earth is half a bathroom?
I e-mailed the seller, asking for clarification, and was sent a detailed plan of the house. It revealed that the master bedroom had an en-suite shower room, plus a small room, which housed a lavatory and a wash basin. On the floor below, there was a room with corner shower,a lavatory and a wash basin and, on the ground floor, a small room, similar to the one next to the master bedroom, which had a lavatory and a wash basin. But, by my calculations, that's 4 rooms, none of which has a bath.
So, I can only assume that the en-suite facilities are collectively referred to as a bathroom, as is the shower room, below, and the ground floor lavatory must be the half-bathroom. But, as I said at the start, Americans call a lavatory a bathroom, so why would this be described as a half-bathroom?
It then occurred to me, that if an American were to receive the details of this house, how on earth, could they possibly know what is meant by the term 'bathroom', as it's clearly arbitrary?
If anyone out there knows the answer, I love to hear it!

[Brownlow]

Thanks, everyone. I loved your responses.
However, I still count the rooms as being 4, in this case; two of which should be half-bathrooms, or three if you count the en-suite shower. In fact, even by American standards, there is only one 'full' bathroom, in the house.
Will we ever infiltrate the psyche of the Americans?

[BenDToy]

Just in case you hadn't realised, Americans also refer to public lavatories as bathrooms, so they go to the "bathroom" in, for example, a restaurant. (I usually find a finger bowl suffices!) or a railway station.
Although more often nowadays they talk about using the "rest room" - obviously to relieve their tiredness!!

[Brownlow]

Hello Ben,
Yes, I remember the restrooms in America. That is the official name, I believe, but people still call it a 'bathroom'. I once even heard a guy talking about going to the bathroom behind a tree. The mind boggles.