Even if you measure one cup of flour exactly how they told you they do it (if they even bothered to tell you how they do it), you will end up with AT LEAST a slightly different amount of flour than they ended up with. Similarly, if you measure one cup of flour precisely the same way every time you measure one cup of flour, you will always end up with AT LEAST a slightly different amount of flour with every cup of flour you measure.
The flour quantity disparity could be at least 10%, even if you're very careful to measure exactly how they measure; even if you are careful to measure the same way every time.
When you measure by weight, 16.5 oz of flour is 16.5 oz of flour. Every time.