There are a lot of things that factor into web browsing battery life...Anandtech, where that chart is from, is probably the most accurate site you can find, as far as scientific tests go (for all subjects, not just battery life).
In any event, I too found it odd that the iPhone on LTE did way better than the 3G numbers...so I went and looked up the iPhone 5 review on Anandtech to see if they mentioned that discrepancy and why it happened like that.
As it turns out, they did explain the discrepency and it actually makes quite a bit of sense. It's also worth mentioning that the Wifi web browsing tests resulted in even LONGER battery life, exceeding 10 hours. So anyway, here is the explanation:
As always we test across multiple air interfaces (3G, 4G LTE, WiFi), but due to the increased network load we actually find that on a given process technology we see an increase in battery life on faster network connections. The why is quite simple to understand: the faster a page is able to fully render, the quicker all components can drive down to their idle power states.
The simple translation - since the page can load significantly faster at LTE speeds, the phone can idle down faster resulting in much less battery drainage. I expect once we see LTE tests of the 8x we will see similar results (longer battery life on LTE then 3g).
For those interested in more reading about the Anandtech battery life bench suite, you can check out the link:
AnandTech - The iPhone 5 Review