Temperature probes take on the temperature of a medium and transfer it to the sensor. To do this, the probe material must first adapt to the outside temperature. For this reason, it is never the temperature of the medium which is measured, but the temperature of the probe / sensor.
The probe requires a certain period of time to settle to the substance being measured. A probe has recognized the temperature of the substance being measured when its temperature is 99 % identical to the outside temperature. This period is known as the t99 period.