Here's what they wrote:

However, with the widely disparate figures we found in the MacBook Pro tests, an average wouldn’t reflect anything a consumer would be likely to experience in the real world. For that reason, we are reporting the lowest battery life results, and using those numbers in calculating our final scores. It’s the only time frame we can confidently advise a consumer to rely on if he or she is planning use the product without access to an electrical outlet.

For the battery test, we download a series of 10 web pages sequentially, starting with the battery fully charged, and ending when the laptop shuts down. The web pages are stored on a server in our lab, and transmitted over a WiFi network set up specifically for this purpose. We conduct our battery tests using the computer’s default browser—Safari, in the case of the MacBook Pro laptops.

No mention they've enabled a hidden developer setting in Safari that can give us a chance to test Safari the same way, if we could reproduce it, then we can immediately tell Apple to fix it.

Once our official testing was done, we experimented by conducting the same battery tests using a Chrome browser, rather than Safari. For this exercise, we ran two trials on each of the laptops, and found battery life to be consistently high on all six runs. That’s not enough data for us to draw a conclusion, and in any case a test using Chrome wouldn’t affect our ratings, since we only use the default browser to calculate our scores for all laptops. But it’s something that a MacBook Pro owner might choose to try.