Drawing a line

From the BBC's coverage of the debate:

Mr Obama sought to draw a line, however, under the appearance of a controversial photograph of him wearing traditional Somali robes during a visit to Kenya in 2006.

Really? Because I thought he was trying to stop talking about it.

Oh. Wait. So did they:

He said he believed Mrs Clinton when she said she did not know where the photo had come from.

"Draw a line under" must mean something different to them. To me it means "emphasize", and can be collapsed into the verb "underline", as he "he underlined the importance of..."

A little Googling confirms that Brits use this to mean "put an end to" and Americans use it to mean "emphasize". (Occasionally "draw a line in the sand" shoves its way in to confuse everybody, but that's a very different expression.)

Does this Brit use come from book-keeping? Anybody know?