You probably don't use a lot of MS products.
Outlook 2000 is 4 (maybe 5) versions old. At least 2010, 2007 and 2003 don't load images by default, I wouldn't be surprised if Outlook XP didn't either.
More to the point: no large organization is using Outlook 2000 anymore. It's unsupported and you can get 2010, 2007, 2003 or even XP on your current license agreement.
... And yet "Outlook 2000, 2003, Express" make up 32% of all email clients found in their study. My point isn't that these are in high use, but that they are vastly over counted (and even then meaningless to try and find providers as there are plenty of people using Gmail inside of Outlook).
They are tracking these based on loaded images, which means Gmail users by default aren't counted. Some versions of Outlook (at least 2000) autoload images. Lots of mobile devices don't load images, which makes BlackBerry users undercounted.
Not to mention sample bias--I also don't sign up for email newsletters.
tl;dr those stats are a hot steaming pile