It is the attitude of mind of the painter that makes the difference.
In the diptych, although the portrait of Richard himself was a likeness, the setting was imaginary and symbolic.

The artist wished to tell in his picture how all the Kings who succeed one another upon the throne of England alike depend upon the protection of Heaven, and how Richard in his turn acknowledged that dependence, and pledged his loyalty to the Blessed Virgin and her Holy Child.

That picture was intended to take the mind of the spectator away from the everyday world and suggest grave thought, and such was likewise in the main the purpose of all paintings in the Middle Ages.

But we are now leaving the Middle Ages behind and approaching a new world nearer to our own.
Hubert van Eyck, in attempting to depict the event at the Sepulchre as it might actually have occurred outside the walls of the City of Jerusalem, was doing something quite novel in his day.

His picture might almost be called a Bible illustration.