Rounding is the difference.
First, of your 100 hands, you say you'll win 20, push 8 and lose 70. What happened to the other two? :D
Using the Dealer Outcome Charts
at this site without rounding:
6 decks, H17, standing with 17vA:
Of 100 hands, you'll win 20.13, push 8.28, and lose the rest 71.59.
The net loss is 71.59 - 20.13 = 51.46 * $10 = $514.60. That's worse than the $500 loss by surrendering.
By the way, if some of you reading this are wondering, surrendering 17vAce is only appropriate when the dealer hits soft 17.
For the really nitpicky, there is some inaccuracy caused by using the dealer outcome charts this way, because they ignore the effect of the two cards making up the player's seventeen.
Slightly more accurate expected values (6 decks, H17):
Standing with T7vA = -0.5138
Standing with 98vA = -0.5117