Nods and shakes

There are numerous differences between UK and USA English.

They're often subtle and usually relatively transparent. One of them is the US convention of referring to the ground floor of a building as the first floor whereas in the UK (and Europe generally) the first floor is reached after climbing a flight of stairs. In other words the US doesn't recognise a floor zero :).

However, the difference that does puzzle me is that here, in the UK, a nod of the head assumes an up and down motion and conventionally implying a consent or a 'Yes' and a shake is a side to side motion - implying a 'No'. I've read in lots of stories here and elsewhere where a character is said to have shaken her head 'yes'. I wonder why the distinction between a nod and a shake doesn't appear to be the norm in dollar land.

Enlighten me :)

R