Confusion
"2deck, s17, ds. .19 adv.
"6 deck, s17, ds, ls, rsa. .26 adv
"I don't understand how 'better' rules are worse for player."
You're not making a proper comparison. If you have better rules and the SAME number of decks shuffled, then better rules are, indeed, better. But, if you have those better rules on a 6-deck game, compared to poorer rules on a 2-deck game, you have to determine which is more important to the basic strategy player: the rules, or the number of decks shuffled.
In this instance, with identical rules, going from 6 decks to 2 decks is worth 0.22% off the top. LS and RSA together are worth only 0.14%, so that isn't enough to compensate for the fewer decks in the DD game.
Clear?
Don