I'd like to point out that those numbers are not market share, they are ownership. It looks like the statistics were done in a bad way since it doesn't account for people owning more than one tablet (for one the 4% other mysteriously disappeared).
Let's say that there are approximately 250 million adults in the US.
According to the data in 2011, 11% of them owned tablets, so 27.5 million (22.275 mil ipad, 4.125 mil Android, 1.1 mil other).
According to the data in 2012, 22% of them owned tablets, so 55 million (28.6 mil ipad, 26.4 mil Android of which 11.55 mil Kindle fire).
Does this sound right?