What does (10 hours)² mean? (specifications) I was looking at the specifications of the MacBooks (pro) on Apples website and I saw this. Built-in battery (10 hours)².

What does the '²' mean? Why (10 hours)² and not 10 hours?
There is an explanation in the footnotes:


  2 Testing conducted by Apple in February 2015 using pre-production 2.9GHz dual-      core Intel Core i5–based 13-inch MacBook Pro units. The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 12 clicks from bottom or 75%. The HD film playback test measures battery life by playing back HD 1080p content with display brightness set to 12 clicks from bottom or 75%. Battery life varies by use and configuration. See www.apple.com/uk/batteries for more information.