Daring Fireball has a good look at the Fun With Numbers you can have in relation to tablet sales numbers.

I took a statistics class in University and the only thing I remember learning in it was that never ever ever ever trust statistics. You can literally make number say anything to support whatever point of view you have, and can do so convincingly. Whoever is tabulating the numbers is in control of the conversation. I can find you perfectly believable numbers that "prove" that Android is #1. I can also find the same numbers to "prove" that iOS is. Or that Ford is number 1. Or Chevy. Or really whatever my bias is. This is the same reason that you have to read the fine print on that late night infomercial that has reports and surveys for their "lose 10 pounds a week all bacon diet".

Of course, Gruber and Daring Fireball is obviously pro-Apple, so you can take that into consideration here. If you're a pro-Android person (and are still reading this) you'll simply dismiss his whole article as pro-Apple bias and that is has no bearing on reality anyway :)