What is odd about it? It seems to be an attempt to get at truthful data.
Yes, according to the graph, on the face of it driving at 55mph is best for reducing air pollution.
The point is - 20mph is a compromise taking into account a lot of other factors.
Maybe it has escaped your attention but 55mph is not a safe speed in many streets and roads?
I realise that many people can't hold two ideas in their heads simultaneously, especially if they are slightly contradictory!
Or to put it simply: fuel efficiency is not the only issue, or 55mph would be obligatory. Neither is safety, or motorised transport would have to stop completely. And fuel efficiency doesn't directly relate to air pollution. There's three ideas together!