To fairly measure the isolation of couplers the most important figure of

merit is the directivity. That is the ratio of coupled output power to leaked

power from the isolated port. so you have to put in evidence that at every frequency in the declared interval the coupling and the directivity are almost the same.

I have not come across the figure of merit for a coupler. Generally, a coupler is specified with insertion loss, directivity / isolation, and return loss.