either you are messing up your final product unknowingly or realizing it's undercooked and having to leave it in the oven for longer to make up the difference.

For frozen pizza/nuggets/etc, the cooking time is always a range. I always just put it in while it preheats and set the timer for the top of the range. Everyone's oven and preferences are different anyway so it's not like some exact science. If they're underdone, I leave them in another minute. Overall it saves time.