Square Enix releases a ton of games. Pre-order numbers will remain at similar percentages regardless of the type of game. People who preorder, preorder, and people who do not, don't. You can get a very good estimate based on that info, then add some padding into the numbers to play it safe. They knew what their max peak capacity was, and where estimates were to come in. If they had done the math right, they would have been ready, by having more servers and/or capacity.