Set an oven to a temperature and the interior spends very little time actually at that temperature. It swings above and below it continuously, and the dial reports the average.
Thermostats work by switching, not by holding
An oven element has one output: on. Temperature control is achieved by turning it on until a sensor reads high, then off until the sensor reads low.
The cavity therefore oscillates through a band around the set point, and in many domestic ovens that band is wide enough to matter for baking.
Delicate items that need a stable environment, such as custards and meringues, are affected far more than a roast, which averages the swings out over an hour.
The sensor is not where the food is
The probe sits on a wall or near the back of the cavity, measuring the air at that spot rather than the air surrounding a tray in the middle.
Radiant heat from the element reaches the top of the food directly and is not represented in an air reading at all.
This is why an oven can be accurate by its own measurement and still brown the top of a tray while leaving the base pale.
The preheat signal comes too early
The indicator responds to air temperature, which rises long before the walls, shelves and door glass have absorbed the heat they need.
Food loaded at that moment sits in hot air surrounded by cold surfaces, which pull heat back out and slow everything down.
Giving an oven a further stretch beyond the beep, particularly with a stone or heavy tray inside, changes results more than adjusting the dial does.
Opening the door costs more than it appears to
Hot air leaves in seconds, but the number on the display recovers quickly because the air reheats first while the surfaces stay cool.
The mass in the oven is what actually cooks the food, and rebuilding it takes considerably longer than the display suggests.
A loaded oven also holds temperature better than an empty one, because the food and trays themselves store heat and give it back.
Every oven has its own geography
Airflow, element position and door sealing vary from unit to unit, so hot spots are individual and remain consistent within one machine.
Fan ovens even the distribution out but do not eliminate it, and they also dry surfaces faster, which is why fan settings are conventionally run lower.
Learning where a particular oven runs hot and rotating trays accordingly compensates for far more than any recipe adjustment.