My guess is that it's what they've been asked to approve and upon which they've done their due diligence. For Netflix Originals, I would assume most production

My guess is that it's what they've been asked to approve and upon which they've done their due diligence. For Netflix Originals, I would assume most productions are done on rental gear. The difference between renting an FX3 and an FX6 isn't going to be significant at production-budget scales. (And if you had a real burning reason to use something off-list I would bet they can write you an exception.)