The Shia do seem to be under repreented
Unless I am mistaken, the Shia are a majority but only had 33 vs 63 Sunni and non denom. They are. I believe, 70% of the Arab Muslims, which means well over 60 Shia should have been polled
I'm not sure about the geographics but the South is primarily Shia and included 29 Shia, so only 4 Shia were polled in the rest of Iraq.
Hmmm, It certainly brings the results into question but I have no idea how it would have turned out with the proper samples. You might assume that the Shia are the most happy to be rid of the Baaths but they are also the least secular.