Why are baking measurements such nice round numbers?
Realistically, there's a lot of flexibility in baking, in spite of the cargo cult mantra that you have to follow pastry recipes exactly. There are simple, weight-based ratios that can be used as a foundation for plenty of variation. The "round numbers" are, in fact, approximations, and this is often why you'll see that a typical baked good recipe in a consumer-oriented cookbook isn't designed to scale exactly past, say, doubling or so.

When you diverge dramatically from a base ratio, you may get an unexpected outcome. But, for example, pate a choux can be converted from the basic cream puff shell to a gougere by simply adding a modest amount of cheese to the dough. Within a moderate range, the exact amount of cheese won't matter that much. When I make muffins, I follow a basic ratio, and add additional items like fruits and nuts without even measuring; given enough experience, you can eyeball how much a recipe can take. 

You can see more evidence of this as you look at most cake or pastry cookbooks. If you reduce the components to their essential character (fat, flour, egg, sweetener, liquid, leavening, flavoring) you'll probably find that there are only about 6-12 archetypal recipes in most such cookbooks. Some of them are even up-front about it and tell you to start with the "basic yellow cake recipe" then add x, y, and z.

That's not to say you won't get different results when you use less or more of an ingredient than the base recipe requires, but you'll probably get very adequate results as long as you're close to the base ratio.

Most chefs aren't particularly scientific, and you should realize that many of our archetypal baking recipes are the serendipitous result of relatively haphazard experimentation. Supposedly, the original souffle was a "mistake" caused by using too many eggs in a cake recipe. Additionally, even with weight based measurements, most ingredients have quite a lot of natural variation, depending on seasonality and, even for things like refined flour, climate and varietal differences can change your results. I've started with some recipes that worked pretty well when I was a student in Germany and had very different (though often acceptable) results in the US with similar ingredients.

Ruhlman's book called Ratio is a good place to look at these kind of archetypal recipes, and includes a fair amount of references on baked goods, though it's not the focus of the book.