PAI uses our own proprietary method to estimate VO2max based on your activity and biometric profile. The main difference with Fitbit (and Garmin) is that they require GPS data to calculate VO2, limiting it to people who run or cycle. PAI, on the other hand, only uses your activity and corresponding heart rates, making our VO2 estimation available to many more people.
Remember that VO2 is only one indicator of your overall health. Activity level over time, regardless of VO2, is potentially a better indicator of health, and therefore risk.