From the website:
"The information on this site covers all cars registered from January 1997 to date. The CO2 value for models being registered since March 2001 is derived from the First Registration and Licensing document. The authoritative CO2 emissions figure will also be shown on the vehicle registration document (V5) from March 2001.
The value for vehicles registered before Janaury 1997 has been supplied by the manufacturers directly to the SMMT's Automotive Data Services (ADS). The value may change dependent on various factors including optional extras and tyre size."