The HC (hydrocarbon)part is what could really be describe as unburned fuel.
On Diesels, the red portion is only 0.3 %, so there may still be some gains in petrol engines
But as most of those 1-5% is already CO , burning it further to CO2 is not going to gain much.
I think VAG missed a bit in this publication though ... page 8
"H2O – Water
is partly induced by the engine (atmospheric humidity) or occurs during low-temperature combustion (warm-up period)."
Water is a normal by-product of burning hydrocarbons ("fuel"), during all combustion processes.
The carbon-part gives the CO2 , the hydro(gen)-part giving water.