I posted this in another thread about Coconut Battery vs. Apple's diagnostic.
"There is a reason for this. The tools that Apple uses looks at the average battery capacity. Coconut Battery and iBackupBot show the real time battery capacity. The battery capacity changes every 5 seconds, depending on usage, charging, and temperature. Apple's tool is more sophisticated and only grabs the average. If you use Coconut or iBackupBot, you will notice in one day it can give you a 20% range. Ive seen as low as 85% and as high as 105% in a matter of 20 minutes. So Apple's tool combines the average and spits out a number."
30% range for a day seems a little high, but not necessarily unbelievable.