so ive had this 13"uMBP for a few months now and no way i ever get 7 hours. im not upset because it still outlasts all my friends laptops. My curiosity is that ive been web browsing for about 30 mins now, also iTunes is open but not running, and thats it. I checked my time remaining for the battery and it was 5h30m. 10 mins later it said 3h30m. 5 mins after that it now says 3h59m. What is causing it to jump all around like this? and is there a way to make it more accurate with its estimates? also. I have calibrated my battery per the directions on apples website, did it about 5 days ago.