My bread recipe book isn't linear with its ingredients?
Mine has only 2 sizes but also has some nonlinearity. You see it with the water as well (in yours and mine). Things like the rate of temperature change, and therefore the rate of rising, will depend on the total quantity, and some change in the proportions of the ingredients will offset that. Yeast multiplies, and a pseudo-sourdough with very little yeast to start with relies on slowly but surely growing the yeast (if you take it to extremes you end up with a true sourdough from wild yeast, with a tiny bit of cultured yeast). So you probably don't have a typo, and can trust the recipe, at least to start with. 

Feel free to experiment; you may prefer the result. I use around 1/3 the stated salt and 1/2 the sugar, for example. The rise and texture aren't very different, but I used to find the bread tasted salty (then sweet if I didn't also reduce the sugar).