What people don't seem to realise is that giving battery life in hours is about the same as saying how many hours your car can drive with a single tank of fuel. How many miles you get out of a tank depends on driving style and road and weather conditions but will be reasonably close; but hours will be all over the place.
That 13" has a 63.5 Wh (Watt hour) battery. It can run 10 hours if you draw 6.35 Watt on the average. Just as an example, I looked at power consumption of a hard drive. Toshiba / Fujitsu hard drives have these numbers:
4.5 Watt while starting the drive (getting up to speed)
2.2 Watt while seeking (trying to get to your data)
1.9 Watt while reading / writing
0.6 Watt while low power idling (whatever that means)
0.18 Watt in standby
0.13 Watt in sleep
If you do the maths: If your Mac uses 6.35 Watt with the hard drive in low power idling and runs for 10 hours, the same Mac will run 8.3 hours while constantly reading/writing and 10.8 hours with the hard drive in sleep mode. That's just the hard drive. (BTW according to TomsHardware, SSD drives are better in idle mode, but not when reading/writing and can actually increase power consumption).
Now this was just one variable. I think the biggest difference is probably the screen brightness. Hard to find numbers, but I found that a Samsung 17" LCD monitor uses 24 Watts - that alone would empty the 13" battery in 2 hours 40 minutes! Then WiFi, Bluetooth, CPU power. The CPU can draw an awful lot more than the 6.35 Watt on its own when running at full speed, like doing Handbrake encodings (useful) or displaying Flash adverts (useless). So it is no wonder that people report battery hours that are all over the place.
Numbers are hard to find. All you get is marketing numbers (like DDR3 RAM is 30% more power efficient than DDR2). I'd want to know how much Watt it takes if I add 4GB RAM to my Mac; it's impossible to find out.
