I'm not going to explain because I do not work at KyoAni, and I'm definitely not going to talk and speculate on something I don't know about and look like a complete and utter fool.
How do you know it took 3 months of preparation? Oh, it's an estimation. I see. A guess. Companies do what the do for money, I don't run one, so I don't know what the method is. Neither do you.