The database covers all vehicles that have a valid tax disc or a Statutory Off Road Notification (SORN). It doesn't cover cars that were off the road prior to the introduction of SORN, or cars that have never been registered on the road since manufacture.
If you can't find your car, it doesn't mean that the DVLA thinks it doesn't exist!
Most vehicles are listed under the make and model recorded on their V5 documents. In some cases though, the make and model name isn't properly recorded and the the vehicle will be counted under one of the following alternative categories:
As with all large government databases, there are errors in this dataset (especially since quite a lot of the data for older vehicles is based on paper records that were originally maintained by local authorities).
The most common error that crops up is vehicles that don't have exactly the correct model variant recorded on their V5 registration document. For example, a special edition Peugeot 205 Gentry might only have Peugeot 205 recorded on its V5. This can lead to some model variants appearing to be rarer than they actually are.
The key to understanding whether or not the data is accurate for your particular model is to check the model name on its V5 registration document. If it's not what you expect it to be, then it's likely that the DVLA statistics for that model aren't very accurate!
This happens more often with older vehicles, especially for those that would have originally been registered in the 1970s and earlier.