Comment by instcode

It can be explained by the fact that Opera isn't the main rivals of Firefox in the browser market.

Edit: http://arewefastyet.com/faq.html

  The team's explanation: Right now, the performance tests
  are run on a Mac, which means no IE. Also the tests rely on
  a "shell" JS engine that runs in a command line. It doesn't
  test browsers. We'll change that, eventually.