It's not so much the lack of lustre, it's actually to do with the wear. For a coin to be classed as UNC there has to be no detectable wear. In this case, there is light but obvious wear on Britannia's right leg, as well as in the topmost points of the KIng's hair. It's not much, but it is enough to take the coin down from UNC status, in my opinion.
Compare it with your 1927, which is UNC, and you'll see the differences.