Why do some instructions tell you how long to preheat an oven for?
There are some ovens out there that beep after a given amount of time, if they've hit the correct temperature or not.

Most of the residential ovens I've dealt with likely aren't at 400F within 10 minutes.  It's possible that the recipe writer wants the dish going in before preheating is finished, but they have no way of knowing how long it takes your oven to heat up, so those instructions are going to give inconsistent results.