Consumption vs CO2
The reason consumption vs. CO2 varies is that diesel fuel contains more (hydro)carbon (and thus more energy) per unit volume compared to petrol. If you burn one litre of petrol you get 2.31 kg CO2, while one litre of diesel gives you 2.68 kg CO2. This is independent of the engine efficiency, which is a measure of how much of the chemical energy stored in the fuel you can extract as power. Higher efficiency = lower consumption for the same work.
All modern combustion engines burn virtually 100% of the fuel, if not they would never pass emission testing (where would the remaining unburnt fuel go?), so CO2 is directly related to consumption within the same fuel type.
While low on CO2 emissions, diesels also emit particulates that are allergenic and thought to be carciogenic, as well as NOx that is poisonous and contributes the the lovely brownish-yellow haze above large cities (petrol engines have virtually zero particulate and NOx emissions).
While particle filters exist (albeit at the price of increasing CO2 through decreased efficiency), a working means of catalyzing NOx is required before a diesel-powered car can be considered as "clean" as a petrol-electric hybrid.