Seems Counter-intuitive to me too.
But I am not going to waste my time researching it since the game can't be beat.
But think of it this way. Take the extreme example. If all the cards were of the same value, EV would be +1100% because three of a kind pays 11:1. If all cards were of just two adjacent values, you still couldn't lose because pairs and adjacents push. So as the card values become more spread out, the EV goes down. The fewer of the edge cards, 2,3 K,A, the more condensed the distribution and the higher the EV.
Just a guess.