Gosh, some don't get the point. What's the difference between a heated oven and a preheated oven?
A pre-heated oven that's just reached 350 isn't the same as an oven that's been at 350 for 20 minutes. Because the pre-heated oven is running hotter as it's still losing heat to the rest of the stove and to the surrounding kitchen. Once the surroundings have heated up, the stove "settles in" and thus you get a consistent cooking temp.