"Generically calibrated" is the opposite of "individually calibrated."
Ambisonic microphones work on principles that depend on the very small differences between their capsules, plus a bunch of other factors. In this case and for the Zoom H3-VR, just because a microphone has four capsules mounted on a tetrahedral mount, that doesn't mean that it will work well.
It turns out that every microphone capsule is different. They might all meet the manufacturer's specifications, but those specifications have ranges of variation, and those variations are incredibly important for ambisonic microphones.
There are also other sources for variations in addition to the capsules.
Calibration is the process that measures the variations and provides corrections for them.
Because it takes time and equipment and manpower to test each of the capsules and then match them into sets, and because it costs money to test each assembled ambisonic microphone to develop correction files for each complete microphone, the manufacturers compromise. They make the assumption that all of their capsules are sufficiently matched, and all of their assembled microphones are sufficiently identical to use a single generic calibration correction.
From the beginning the assumptions are wrong. And as the microphones age, the assumptions get even worse. You're still stuck with the original generic calibration.
That's why each ambisonic microphone needs to be individually calibrated, and supplied with its own correction file. And as it ages, it will need to be re-calibrated.
Ideally, each set of capsules should be matched for a bunch of specifications - not just frequency response and/or sensitivity. And then when they're assembled into the microphone mount, each microphone should be measured so that its performance can be corrected with an individual calibration file.
All Core Sound OctoMics and TetraMics are individually calibrated. We recommend re-calibration every two to three years, depending on application. We offer a recalibration service for our microphones.
(We also offer an initial calibration service for other manufacturers' ambisonic microphones, including those from Rode, Sennheiser and the old Soundfield.)