Actually.....I took a closer look, and it seems indeed that they never confined the choices to separate "categories" until this year.
Here's the article from last year (Jan. 2004 issue):
http://www.caranddriver.com/article.asp?section_id=33&article_id=7571
.....and here's the current one (Jan. 2005 issue):
http://www.caranddriver.com/article.asp?section_id=33&article_id=8914
If you click on the arrows of that box near the top of this year's article where you can
"choose one" of the 10 Best, you can see that each car is representing a category.
If you click the same on last year's list, that's not the case; it's just a list of 10 cars.
That doesn't mean for sure that they didn't observe the categories, but I'm pretty they didn't. For example: Look at what happened with the Toyota Prius. It made last year's list, but not this year's. If you look at the categories for this year, it doesn't look like there really is any category for it; the only one that would even possibly fit would be "Family Sedan," which was won by the Accord.
And last year, both the Accord and the Prius made the list.
Unless I'm missing something, they don't state outright that they changed their system. If I'm right that this was a change, they really should have made it very clear, because it would help explain why some cars got bumped off. There's nothing suddenly wrong with the Prius, and it's not like it got "passed" by the newcomer cars; it's just that they don't put it ahead of the Accord, and they didn't necessarily last year either.