I bought a new battery for my Macbook. Besides not giving an accurate indication of the battery (due to a calibration issue I guess), it gives a different value to the Mac and the coconutBattery. What could be the reason?
Coconut Battery and Apple use different methods to calculate the percentage.
The Coconut Battery FAQ says:
iOS uses a defensive calculated battery capacity value to define 100% (and the percentage calculation). coconutBattery reads out the raw battery data and can give you more precise information. (for example you will see that the device actually is still charging when iOS tells you that it is fully charged).
(It says iOS, but the principle is the same.)
A quick Google shows lots of people asking this question.
Any figure is going to be a rough guide rather than a measurement that you can use in any accurate determination of anything.