As an American, your question made absolutely no sense to me, because I had never heard of DVLA or V5's, but I was able to find the answer:
"CO2 Information - The CO2 figures shown are representative of the vehicle tested and may vary between specifications (variants/versions) of a given model. As such the figures are indicative only. A definitive figure for a given specification (variant/version) will be available at the point of sale."
Variant and Version are referring to the specifics of the automobile. For instance, there may be two Aston Martins, but one is DB7 coupe with automatic transmission and one is a DB7 convertible with a manual transmission. Coupe/Convertible and Automatic/Manual appear to be the Variants and Versions referenced. See more info here:
http://www.vcacarfueldata.org.uk/e_howtouse.htm