You bring up logical points but I’d imagine Gartner considered all of those factors. They usually do.
The most likely scenario for iPad sales this year is 4.2 million units," says Gartner analyst Angela McIntyre
Source dated May 5, 2010.
Research firm iSuppli raised its prediction for the number of iPads that Apple will ship this year from 7.1 million to 12.9 million.
Source dated July 20, 2010
300% error in 2 months. At least they didn't predict it would fail monumentally. I expect that this number will be adjusted upward again, as Apple has increased production to 3 million units a month as of Oct 4th. I was one of the lucky pre-orders who received it a day earlier than promised. The polish put into that product cannot be understated. I use it daily, and am amazed by the amount of thought that went into it.