If say on my drive yesterday from Brum to London and back (M40, M25, into London on the M4, 2 stops, out of London on the M11, M25 then back on the M40) I were to have been driving at about 90mph on most of the motorway, that plus driving slowly through London I would have expected my display to say 43mpg.
On UK roads however that would be illegal so my display didn't/wouldn't have said that.