OK
The battery contains a certain amount of power. Measured in milliamp hours (mAhr). That's so many milliamps of current for so many hours
When full your battery contains eg 6000 mA hr. So it can power something needing 6000 mA for 1 hour, something needing 3000 mA for 2 hours etc. Or something needing 600 mA for 10 hrs.
Using a car analogy - your gas tank contains 10 gallons. That will drive you 300 miles at 70 mph, or 100 miles at 120 mph.
The % figure is the amount of the 6000 mA hr which is left in the battery (amount of gas in the tank). The Mac knows that figure pretty accurately, because it can keep track of what goes in and what comes out (pretty good gas gauge). So if you have 5000 mA hr remaining, it will read 80%.
But how long will 6000 mAhr last you? It depends how much power your Mac uses. And that varies a lot from moment to moment. In particular your CPU can use anything from about 500 to about 3000 mA depending on how hard it's working. Add in other components (screen, WiFi etc) to get 700 - 3500 mA. Note that's anything from about 10 hours to <2 hours depending on what you're doing.
Every 30 second (?) your Mac looks at how many mA the CPU and other components are using. It then divides the power remaining in the battery by that figure to get a runtime. For example, let's say your CPU is completely idle. Your Mac might use 700 mA. If your battery is full with 6000 mAhr in it your runtime will be 8.5 hours. But let's say 30 seconds later, Firefox is working your CPU harder and your Mac is using 850 mA, runtime will drop to 7 hours.
The truth is, your Mac has absolutely no idea how long your runtime will be - because it doesn't know what you are going to ask your CPU to do in the future. When it says "runtime 8 hours" it means "if you carry on doing exactly what you are doing now, you will get 8 hours". But if you start gaming, you will only get 2 hours. Because CPU load jumps up and down from second to second, so does your runtime.
Calibration only affects the % figure - it give the Mac a better idea of how much power can fit in the battery. Calibration does directly not affect the runtime figure.
It's just the same as your car. If you have one of those fancy driving computers it will predict how far you can drive on the gas in your tank. But only if you drive like you are now. If you start going up a steep hill, it will change the range prediction a lot. When you go down the hill the prediction will go the other way.
You can see how much power your Mac is using by opening System Profiler and clicking on Power. Look at the Amperage figure. Every 30 seconds that figure will change (but you have to press Cmd-R to refresh it). I expect you will see that amperage figure bounce up and down a lot as your CPU load changes. To get the runtime figure I believe your Mac just divides Charge Remaining by Amperage to get hours runtime.