i came to this forum as my battery health had dropped from >90% to 74% within 24h. i unplugged it just an hour ago, when the battery was at 21% - and when i unplugged it, it instantly dropped to 4%. that's how i noticed the battery health was @ 74% (iStatPro). however, after plugging it back in, leaving it for half an hour (being used tho), and i then unplugged it, the battery health kicked back to 92% and the battery charged actually increased (just by unplugging) from 20% to 34%...
so it seems to me to be some kind of problem with the reading of the battery health/charge - and perhaps indeed calibration is a good way to go forward. (as it seems it's not just iStatPro that gets confused, but os x as well.)