aren't carnivore diets quite monotonous in nature anyway?

There's probably seasonal variation-- Farley Mowat described wolves eating a lot of mice during the summer when mice are plentiful. Also, I'm pretty sure carnivores eat the stomach contents of their prey-- more seasonal variation. And in temperate-to-cold climates, prey will have the most fat in the fall and the least in the early spring.

It wouldn't surprise me if there's a nutritional variation for dry season/rainy season climates, but I don't know what it would be.