All countertop oven settings are off. It's a flaw in the design. the temperature will vary with the surrounding (room) temperature. they've been known to be off over 75°
Place a couple thermometers in the middle of the shelf you will bake on and adjust your oven setting until the thermometer reach 400° (IF the thermometers don't agree with each other borrow another thermometer and keep the ones that agree with eachother) It is good to check the setting every time there is a big change in the surrounding temperature.
Another thing that will help is to cover the bottom of your oven with heat retaining media I use ceramic BBQ briquettes, other use firebrick, chunks of metal, ??? this will help the oven return to 400° faster after opening the door.