A lot of that will be down to the fact that the most popular android tablets are wifi only. The iPad sells a lot more on contract, thus has higher web traffic in general.
The iPad is also still a fair bit easier to use as a web browser, especially with Chrome.
Web traffic doesn't tell the full story now does it.
Many people buy tablets to just read and/or play games. Games that don't necc need the internet. They also use them to work on documents. Again - not necc requiring internet.
I've never understood why so much "weight" is put into internet usage as "proof point" that people don't use their android devices and/or they use Apple devices more.
Keep to the facts. People access the internet more on Apple devices than Android (for example). That does not equate to whether or not they use their device more or less than the other.