Apple Thoughts: Apple Market Share 8%, or is it 6.4%?

"Quarterly PC shipment estimates were released Wednesday, and Apple's share of the U.S. PC market grew 34 percent year over year to capture 8 percent of the total domestic market."

So says Gartner, however IDC says the growth was actually 8.3% and only shipped 1.130 million versus Gartners estimate of 1.39 million units. IDC also differs on the Market Share, estimating Apple's market share at 6.4% versus the 8% that Gartner gave them. I would have thought they would base their estimates on similar calculations, aren't they querying the same vendors? I have to admit my ignorance for how these numbers are calculated, any thoughts?