mark-up and gross profit ratio

Hallo, I have the following example, and am trying to understand the answers: The Port Elisabeth fishmonger and the Port Elisabeth bookseller both operate on a 50% mark-up on cost. However, their gross profit ratios are as follows. Fishmonger 25% Bookseller 33% a) there is more wastage with fish stocks than with book stocks A: The fishmonger’s gross profit margin may be low because of wastage My Q: How is wastage related to gross profit margin? Is it because we will have more lost inventory or higher inventory cost, consequently, Sales - CoGS => GP will be less? b) the fishmonger’s turnover is declining whereas that of the bookseller is increasing A: declining turnover would not directly affect the gross profit percentage My Q: Why doesn't the declining turnover directly affect the gross profit percentage? The turnover is included in the calculation of Gross profit, it is not possible that a change in it, will not affect the gross profit percentage, or at least I don't see how it will not affect it. Thank you!