Either the usage share numbers are wrong, or people just don’t use the Android tablets they buy.
I wonder if they include non-Google versions of “Android” in these numbers. But in answer to John’s query, if you were not entirely committed to tablet computing, wouldn’t you be likely to buy the cheapest tablet available? And when the user experience doesn’t wow you, you tend not to use it. It’s obviously not like that for everyone but I wonder if that doesn’t explain some of this.