I know. It's just that in that particular case (going one-on-one with Aegislash), I was more comfortable having Zygarde with an Impish nature. There is a small chance that Aegislash can 2HKO Zygarde if it has a Careful nature instead. It's a more solid counter to Aegislash specifically if it runs a +Defense nature.
But you are right, in almost all cases, a Careful nature is much better for it.