"tag question" vs. "question tag"

I've just read this page: http://en.wikipedia.org/wiki/Tag_question

So regarding this passage:

The term "question tag" is generally preferred by British grammarians, while their American counterparts prefer "tag question".

If a British person uses the term "question tag", does it refer to the American term "tag question"? Is this passage saying the terms are synonymous?

As far as I can tell, "tag question" is a term referring to the "question" and "question tag" is a term referring to the "tag" itself, which means they refer to different things and are NOT synonymous. So why does Wikipedia say that the term "question tag" is generally preferred by British grammarians, while their American counterparts prefer "tag question" when they in fact refer to different things (which means that both British and American will have to use either depending on which term they are talking about)?

"Here's a question tag" means "Here's a tag, and the tag is the question tag".

"Here's a tag question" means "Here's a question, and the question is a tag question."

So are "tag" and "question" synonymous? If not I can't see how "tag question" and "question tag" are synonymous.

Technically, the part after the comma is a sentence for itself. If you say "question tag" or "tag question" you must refer to the second part of the main sentence, the subordinate sentence.

In this regard the both phrases mean the same.

The terms are synonymous. Superficially at least, this looks like the difference between "color" and "colour" - that is, different, but not in a way that changes the meaning.

As for why there's a difference, I've had no luck finding any official explanation, but it sounds like there may be a slight conceptual difference - in "tag question" the emphasis is on the "question" (which is modified by "tag"); whereas in "question tag" the emphasis is on the tag. So perhaps there's a difference between how they are thought of - as a question that has a tag on it, or a tag attached to a question.

I went to a local library and found a grammar book for upper-intermediate students from Oxford University Press, so I thought a picture might worth more, etc., just go to @teacher2go to watch it.

Here it is what it says:

We use tag questions to query a statement.

A: She's Russian

B: Is she? I thought she was German.

So the statement is queried by the tag question in the same affirmative form, and not negative, "is she?"

Tag questions are similar in form to Question tags. However, we use an affirmative tag question after an affirmative statement, and a negative after a negative statement.

A: It's nine o'clock.

B: Is it? I should go home.

A: I'm not ready!

B: Aren't you? Well, then hurry up!

As I said before, it is not a matter of being American or British. By the way, it is a British grammar book I found.