Realgas always exerts lesser pressure than the ideal because of

Initially it was assumed that gases follow the laws of ideal gas but later it was observed that at low temperature and high pressure gases deviated significantly from the ideal nature.

This was first accounted for by Van Der Waal

In case of pressure, he observed that, one assumption of the Kinetic Molecular Theory was incorrect i.e There is no force of attraction or repulsion between gas molecules.

It was observed that there is some amount of intermolecular force of attraction between the gas molecules namely the van der waal forces.

This doesn't let the molecules hit the wall of the container with full force as it is held back by attractive forces.

This is why, due to the force of attraction, real gasses tend to show slightly lower pressure as compared to ideal gasses.

P ideal = P real + an^2/V^2

where, 'a'(constant) is the measure of magnitude of attractive forces among the molecules of gas. ‘n’ is the number of moles present and 'V' is the volume of gas present.