Why is every percentage against net revenue?

So they can be compared and added. A set of percentages with different denominators cannot be reasoned about: a cost measured against gross revenue and another against net revenue are two numbers that only look like they belong on the same page.

The four exceptions are deliberate and labelled: price reductions and discount codes are measured against the ticket value of what was ordered, returns against gross revenue, and the tax rate against what tax was actually charged on.