I think what you are missing is that 9.4% is an aggregate number of ALL personal computers sold in the US including desktops, notebooks, netbooks and servers. NPD's figure is citing 48% of the Revenue of only DESKTOP computers sold in the US sold at RETAIL. That excludes notebooks, netbooks, and servers AND all computers that are not sold at retail... which would exclude sales to governments, etc. NPD also reports that Apple recieves 33% of the revenue for the retail sales of notebook computers (apparently including netbooks).
In addition, someone has pointed out that apparently Apple may actually have hit 17% of units sold in October since the new 21.5" and 27" iMacs were just released.
That could be it, although I wonder if they’re selling that many servers that it distorts the total computer figures so much.
The multiple computer household numbers offer a more interesting picture. Some of these are homes with one machine for mom and dad, perhaps another machine that one of them uses for work, and then a pile of Dells for the kiddies. These are the Bic lighters of the pc world.
But the multiple machine household also includes all the people who explored the limits of the pc and decided to see what Apple systems offer. IMO this is the track for the typical Apple user. He gets a pc, moves through the more advanced systems, then decides to see why these other folks are so interested in Apple. One of the references that I read derided the claim that only 2% of households exclusively use Apple machines.
It seems to follow a typical growth through affluence path. The user starts with an old pc, like driving his parents’ old Ford, then his income grows and he gets a new Ford, then a Lincoln, then he moves into a Mercedes or BMW. This also reflects the fact that the Apple users tend to have more wealth.
Like I said, my first computer was a Mac, and I only left the platform because of software availability. Now I’m not looking forward to the investment I’m going to have to make on Apple systems, and that is a knock on the pc. PC systems just won’t fill my future needs.