Thanks once again for your very interesting computer tests, Mr. Muller!
Notice that if this were simply a matter of the K capturing moves being unusually strong and the K non-capturing moves being unusually weak, we should expect mKcN to be weaker than K, but (if I am reading you correctly) the tests say it is stronger.
So if you are correct that K's moves have a higher 'ideal value' but are suppressed by K's overall low speed (which would have been my first guess also), then we must conclude that a piece's 'speed' depends significantly upon its capturing moves, not only its non-capturing moves. This seems surprising to me, because I would imagine that capturing capabilities are only exercised in a small percentage of the piece's movements.
Alternately, this could be a matter of special synergy rather than a global bonus or penalty--that is, perhaps the combination of speed and concentrated attacks is particularly valuable, but neither component has significant value by itself. Thus, the mKcN, lacking both, is not significantly worse off than either N or K, which only have one each, but the mNcK stands tall with both.
Curious.