First thought: as a curser, Sea Hag is good at breaking up combos, so its strength will be higher when Fool's Gold is present. Since a SH/FG opening guarantees FG is present on the board, whereas with SH/Silver you don't know, that would influence the rankings.
Second thought: In Sea Hag games, it's pretty hard to hit $5. FG makes it easy to hit $5, because it happens whenever you get two of them in hand.
(But neither of these reasons explain why YW/FG is ranked lower than YW/Silver... possibly because YW/FG openings occur most often when FG is bane, which decrease the strength of YW?)