Take a look at how the vacuum oven is made and how heat energy is transmitted in a vacuum, to best understand why adding shelves with thermal mass helps stabilize the temperature.
For starters, the oven only has heaters on a couple of sides of the shell, so it has to soak a long time for the heat to transfer to the two unheated sides. The locations where the heaters are attached, get hotter than those where they are not, making local hot spots.
The control thermocouple is attached to the oven shell, not inside the oven or to the shelf, so the temperature reading has only slight bearing on the temperature inside.
Heat can transfer to anything inside by radiant energy alone. That is not as efficient as convection and takes much longer to heat up what you put in the oven.
A vacuum oven at solid state, with thermal mass in its shelves, has already heated up the shelves through radiant energy, and they don't lose heat fast when you open and close the door, so the oven doesn't have to add as much energy to again stabilize at set point, so it doesn't overshoot as wildly.
When you turn a heat strip on, it continues to get hotter until it trips the set point at the location of the thermocouple, and that isn't at the strip heater location, so the strip heaters and oven shell that they are attached to are much hotter than set point by that time.
They also have stored energy, so that when they are turned off, they continue to get hotter for a bit afterwards, hence control overshoot. A PID controller attempts to better control the temperature, by ramping up to the point where the controller shuts off energy to the strip heater. It does that by pulsing on and off at a slower rate as it approaches set point, starting ten degrees or so away, instead of just on or off at set point.
The self learning controls also keep track of overshoots, etc and refine control dead band all by themselves. There are however limits to their resourcefulness, and helping them by taking out the major swings, allows them to narrow the dead band.
