An erroneous assumption methinks... I bought one litre for £9... which is true... however also true is that I bought, at the same time, a second one for the same price. This way of stating a price means that one can deduce the price per litre in less words.
For example... If I had said 'I bought two litres for £18'... one would only know the price I paid for 2 bottles... it could have been that there was a buy one get one free deal.. or buy one get one half price... therefore one could not be certain to accurately infer the price of one litre without asking another question.
That is why it was not odd and delibrate.
Yes... i'm bored!