In the SpaceX Demo 2 Crew Dragon livestream, they had to equalize pressure between the ISS and the Dragon spacecraft. The voice on the stream said that they were bringing it to levels that match Earth's. What is the pressure in the Dragon if not exactly that of Earth's? And why would that be the case? Is it intentional or something that just happens.
The pressures of both the Dragon spacecraft and the ISS match the atmospheric sea level pressure of Earth, about 1 bar. But there are small inevitable tolerances of about some millibar or less. So there is a non zero pressure difference between the spacecraft and the spacestation.
When air temperature in the closed spacecraft changes by only 0.3 K, the pressure would change by 1 mbar.
If we assume a hatch with an area of one square meter and a pressure difference of 1 mbar = 1 hPa = 100 Pa, the resulting force is 100 N. On Earth you would need about 100 N to lift a weight of 10 kg.
To remove those small pressure differences a valve is opened to align pressures on both sides before opening the hatch.