The effect in play here is card-strength, and how that ADVERSELY (relative to weaker boards) affects top player win-rates.
Strong players have a BIGGER edge on non-obvious boards. When there are harder choices to make, the better players will make the correct call more often than weaker players. However, for boards with Montebank or Witch, it's almost always (baring something like masquerade or ambassador) correct to rush for the powerful $5 attack. This is obvious, even to a lesser skilled opponent. On boards like this, the better player just lost some of his "edge".
So this lesser skilled opponent is quite lucky when he opens a kingdom with some of these game-defining cards. Their tough decision on the intial buy is essentially made for them, and this is one less area that they might get outplayed this game. This is GOOD for the worse player of the two, and BAD for the better player.
The top players would get immensely more "edge" on non-obvious boards, where their higher knowledge/experience would lead them to buy correctly more often than a non-elite player.
TL:DR... this stat really only tells me that in games with powerful cards, the powerful cards dominate. In really whacky boards, the top players would outplay a slightly worse opponent more-often. I.e. if I play against theory, I should be happy if I open an obvious witch-dominated game.. because half my decisions are pre-made for me, and I have less room to make errors against such a strong opponent.