We all know what a measuring cup is.
The issue is that flour is quite compressible. So if you've opened a fresh bag and scooped out a cup from the top, you'll get quite a different amount than if you dig in the cup and compress the flour against the side of the bag.
It's not like scooping out rice or sugar, where a cup is a cup, assuming you're careful to use a measuring cup and fill it exactly level.
I'm not sure of the exact error margins (there is research on these things though if you care to look) but at a guess I'd say you should expect 15% error margins on scooping white flour even if you're extremely careful to fill the measuring scoop to the same level each time. Probably a bit less on heavier flours.
Note, I run a bakery, although I'm not a baker myself. But I am the one focused on making sure we have repeatable processes, as much as that's possible for sourdough!
Volume measurements are extremely innacurate and should be avoided for pretty much everything in baking, except maybe water. Not a big deal if you're baking at home - bread with +-15% flour will be totally edible - but if you want repeatable bread quality, use a scales with accuracy to 0.1g. You can buy one for $20 that's good enough for home use.
If you're going to weigh even one thing, weigh the salt. Small variations can have a big effect on yeast activity and final bread flavor. Volume measurements of salt are not at all accurate because crystal sizes vary a lot. People assume (even some bakers that I've met!) that salt is just added to bread for flavor, but it's more like a chemical reaction rate control dial, and you need to be very accurate in how much you turn that dial.