If you stick a temp gauge into the toaster oven you can preheat it to 350 as you open the door and stick a cold tray into it filled with coolish bullets the oven temp will drop. Just open the door and it will drop.
Then you get a full on by the elements your temp can get much higher than the 400 and then back down and then back up as it cools. even a big full size kitchen oven will do the same.
I stuck a thermometer into my oven and watched it . My 350 setting works well.
Not all ovens will be 350 when on 350 it is just a set of points on a steel strip as the strip heats up it bends and breaks contact no more power to the element until the strip cools and the points contact each other.
I have fixed a few electric griddles and deep fryers that would not get up to more than 250 degrees even though the dial was maxed out to 400+
Just bent the steel strip a bit.
One mans 350 is another mans 406