Good question. There are two major parts to our process.
1. Adding device, browser and operating system details profiles. For example; the HTC Sensation.
2. Pairing HTTP headers to these profiles.
New profiles are added as soon as the manufacturers provide sufficient details.
Everyday we receive millions of new HTTP headers. We pair these headers against the profiles automatically where we have defined a robust rule for doing so. Sometimes new format headers are presented which we've not setup a rule for and these are processed manually, ideally by creating a new rule once a common format has been identified. Headers with the highest frequency are reviewed first, or where a developer raises an issue with us we'll look at it straight away. This is what happened in your case.
We also use real devices and 3rd party testing services to validate against.
At the moment our tests indicate the data set is 99.8% accurate and over 140k web servers use our services.
In summary a combination of our process, real devices and users reporting issues publicly via these forums provides transparency and confidence.