When I bought mine (18 months ago) it was the only van-based car that was in the £30 road tax bracket. Everything else was £130 and above. Some of the latest ones are £20. I don't know whether the others have caught up yet.
CO2 is completely related to fuel consumption, which is a combination of the driver and the vehicle.
There's also the Nox and soot, but they're all pretty similar on that score - they all do just enough to pass the tests, with or without faking it.
So the Citroen was the best of a very bad bunch then.
The problem is probably the limits themselves - if they give impossible and conflicting targets this is what they get. Nox is produced by efficient engines, as it results from having too much oxygen and not enough fuel - squirt plenty fuel in and none would be emitted. But that would lead to a high CO2 output.