I always have trouble pouring coffee or cocoa powder. The containers are never designed with a spout so I use a spoon or pour it from tiny plastic container  and my fine motor stability isn't fantastic. How can I avoid spillage? 
Who’s right? In a way, all of them are. The weight equivalent of volumetric measurements will depend on the packing, which in turn will depend on the baker. The probably lowest value you will get if you use slightly older flour, stored in a dry environment, sieved, then spooned into the measuring cup and leveled. That’ll be pretty close to, I’d say 110 g. In contrast, a flour from a humid environment, that has settled during transport and storage and was then scooped directly with the measuring cup - you can get easily close to 150g in that case. Admittedly, the latter isn’t good practice, but it shows the difference and the Achilles heel of measuring by volume. When a recipe requires precision and reliability, it’s usually written in weight-based measurements and ratios. In daily use, many recipes have enough tolerance built in for the kind of errors introduced by volumetric units, and some will explain the method (e.g. “spooned and leveled”). In baking there’s often a final step that says add “1-3 tablespoons of milk” or “as needed” - aiming to get the batter to the desired consistency. If you need true precision, use a scale. There are e.g. macaron recipes that start with weighing the eggs, then weigh the other ingredients based on that. Likewise commercial recipes that will list “350g egg” instead of seven eggs - which is roughly the same amount, but not necessarily exactly the same. 