This is what the website says about the data ..
"How far does the data go back?
The data officially goes back all the way to the year 1900. However, the completeness and accuracy of the data gets progressively worse the further you go back, especially in regards to colour.
How accurate is the data?
As with all government data sets, there are errors. Most commonly, vehicles are registered under slightly incorrect model names. To be sure you're looking at the correct data for your car, it's worth cross-referencing the model with what it says on your V5 document"
Recognising the fact that data accuracy isn't 100% and the website itself simply purchases data fields from the DVLA.