Canadian Study - P2P Users Buy More Music?

From the study:

Among Canadians who engage in P2P file-sharing, our results suggest that for every 12 P2P downloaded songs, music purchases increase by 0.44 CDs. That is, downloading the equivalent of approximately one CD increases purchasing by about half of a CD.

However; as P2P file sharing is increasing in the US, then why aren't US music sales also increasing?

From NPD Group, 3/14/07:

While P2P user growth rates slowed, the average P2P user downloaded many more files in 2006 (5 billion files) than the previous year, which represents a 47 percent increase in P2P downloading compared to 2005 (3.4 billion files).

Yet, according to the Pali Research WMG report (subscription required) of 11/01/07; over the same period 2005 - 2006, US combined physical and digital music sales fell by 1.2% and physical sales alone declined 7.8% (Soundscan data).