Flour's ability to absorb water changes depending on what variety of wheat and where it was grown and what the weather was like during the season.
Weight is also just a guideline. Baking is not an exact science.
Pretty sure any pastry chef will strongly disagree with that. If anything, baking is the cooking activity most akin to an exact science. The amounts need to be carefully measured, the temperatures need to be exactly right (e.g. Italian merengue), the baking time needs to be correct to the second for some dishes (lava cake).
Yes, the measures can change based on the flour or its substitutes (ground pistachio for example), but the processes involved require an equal amount of precision.
A lot of chefs call cooking an art, but baking a science.
I am a former pastry chef and baker. You'd think it's very precise work but it's actually mostly intuition based on experience. You know the recipes and tweak them as you go.
Also the batch sizes are many times bigger than a home cook ever makes so a cup of flour more or less usually makes no difference to the end product. With leavening agents the margin of error is smaller obviously.
This whole thread is pretty triggering to me. People think that if the recipe is exact enough, it'll come out perfect the first time and they won't have to make any tweaks sure to their ingredients, their equipment, or the environment.
There's a reason why I generally won't make a recipe for the first time for guests.
And weather/storage. If flour is stored in a humid environment in a paper bag (like on a store shelf), it will get heavier as it takes on water. This messes up the weight of flour but also throws off the amount of water in the dough.
That said I prefer baking by weight, not because it’s more precise, but because I don’t dirty dishes for measurements.
If you know the factors that affect the flour, you can control said factors, thus predict your results based on such factors, more or less a measurable margin of error. Ergo, baking is precisely an exact science.
Random sampling flour batches. And you'd think I'm joking. But no, this is exactly how we invented cookies. Cookies were baker's experimental tool to test their flour and, by ovserving the cookie, predict what they needed to change in their bread recipes to produce the exact result they wanted.