I think what they may be referring to is mothers who bf for all or some of the first year, ie about 2/3 of women in the UK initiate breastfeeding (actually I thought it was slightly more than that, but I think 2/3 is not far off, is it?)
They don't mean the number of women who partially feed for one year, they mean the number of women who feed for all or part of that time.
But I agree the sloppy phrasing makes it ambiguous.