My bread recipe book isn't linear with its ingredients? The book that came with my bread maker machine has bread ingredients and quantities in the back but I note that they aren't very consistent in terms of quantities. Take a look at this sourdough:



I note that the 1.5lb loaf uses the same amount of sugar as a 1lb loaf, but a 2lb loaf uses 3x the amount of sugar as a 1lb loaf

I'm hence wondering if I should use 2tbs of sugar, this being the halfway point between the 1lb loaf's 1 tbsp and the 2lb loaf's 3tbsp? 

Similarly they all use the same amount of yeast; does that seem right? By that pattern I could make a tonne of sourdough using only 1/3tsp of yeast! :)
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).