[Gys]

Authors explanation:

'It seems like Microsoft has been targeting their optimization effort for the competitor benchmarks in order to show impressive results for their new product. When it comes to more intensive and complex HTML5 benchmarks they are still miles behind the competition.'

Reasonable explanation to me.

[Joeri]

I think instead they worked in JavaScript performance, so in js benchmarks like sunspider and octane they do well. Meanwhile their page loading was not as optimized, so benchmarks that test loading whole pages such as peacekeeper don't perform as well.

[Gys]

Peacekeeper itself says they focus on js.

http://peacekeeper.futuremark.com/faq.action:

Peacekeeper measures your browser's performance by testing its JavaScript functionality. JavaScript is a widely used programming language used in the creation of modern websites to provide features such as animation, navigation, forms and other common requirements. By measuring a browser’s ability to handle commonly used JavaScript functions Peacekeeper can evaluate its performance.