However, overall Asia contributes 50.1%, Europe 21.4% and North America 14.2% (presumably the other 2.9% comes from Canada and Mexico).
None of this makes any comment as to the actual origin of the spam, it is merely a general assessment of the numbers of computers taking part in the various botnets being used to send the spam - the owners / controllers of these botnets could be anywhere in the world.
With all this in mind, there are a couple of conclusions.
Firstly, the number of spam-transmitting computers reflects the overall computer population in each region, but it also reflects the preparedness of the computer-owners to combat attacks upon their machines. The vast majority of non-licensed Windows computers are to be found in Asian countries; these machines are also likely to lack even rudimentary security protections.
Secondly, as the Sophos report suggests, the best thing any of us can do is to NOT buy anything via a spam message.