[Buzz]

Tim - Good eyes! I think it's just a typo. No big deal. I get 19, the same as you, and make it a total of 27 cards on the river that could possibly make a straight or flush for an opponent (without also making Hero quads or a full house). That leaves only 17 favorable or safe cards, and probably for only half the pot.

But Hero's hand odds aren't quite 17 to 27, because of two opposite effect factors.
(1) There's a slight chance of an opponent making quads on the river.
(2) Depending somewhat on the exact river card, there's a fairly good chance nobody will actually make a straight on the river, even if one becomes enabled.

Buzz