not necessarily. For a very general view yes you are correct but there are instances where this is not the case. For example both TF and Brioni have 32 inch trousers for their 38 (48Reu) jackets but TF is much slimmer fitting. Although im not quite sure that the "drop" refers to the difference between jacket and trouser, it might refer to the jacket chest size vs jacket waist. Even within the same brand for example Brioni, i barely have to take the waist in on some models but have to quite a bit on others. Qunirile vs Nomentano comes to mind for this case even though both are 38r jackets with 32 pants
Of course you are right that "drop" alone is just a general guideline and there are many exceptions. But "drop" is definitely the difference between the jacket chest and trouser waist, not between jacket chest and jacket waist. The difference between jacket chest and jacket waist is a smaller number.