The iPad having 97% of tablet Web traffic is intuitively a far more accurate number. Just think about how many tablets you see in the wild and how many of them are iPads.
As for why: I think the answer comes down to the ecosystem being far more important on tablets than on phones. Any smartphone can send and receive calls, send and receive text messages, send and receive emails and use some variety of maps. IMHO that covers the vast majority of most people's usage.
But on tablets? Games matter. Apps matter. Far more than on phones (IMHO), hence Apple's unrelenting dominance of the space (so far) and also why people expecting a repeat of Android's rapid growth in the phone space just don't seem to understand the difference. But I guess that's the danger of analogies.
Android's Kindle Fire is (IMHO) the first serious iPad competitor to emerge and the first non-iPad that will get remotely serious (>1M units) marketshare.