Absolutly, you cannot trust statistics, they can only ever be a rough guide.
I myself have one purchased copy of Linux, two downloaded distros, and I am about to give out two copies of Red Hat and Two copies of Mandrake, that means that for one purchased copy of Linux there are 6 more that is downloaded and/or copied.
Now if this ratio followed through all over then you would end up with you're 1.7% being a stomping great 11.9% (theoretically beating Macs).
So in one simple piece of fudging the figures I have made Linux outuse Macs as a desktop O/S.