A strange statistic

'In 2006, there were 26.6 million (Alzheimer's) sufferers worldwide.' [Wikipedia]
This figure, or one close to it, is quoted time and again. Yet if 1 in 3 of us will develop AD, the number of sufferers worldwide must surely be vastly greater than 26.6 million. Now I can think of a couple of reasons why the proportion of sufferers in other countries might be smaller. In many developing countries people die, on average, much younger, and as Alzheimer's is predominantly a disease of old age most people probably die long before they develop it. Also, we know that the condition often goes undiagnosed even in our relatively developed country so it must be hugely underdiagnosed elsewhere. Even so, I would be very interested to know how the figure has been calculated and whether there are any other explanations for it not being very much higher.