The pressure of real gases is less than the Pressure of an ideal gas because of:

${\mathrm{P}}_{\text{Ideal }}={\mathrm{P}}_{\text{Real }}+\frac{{\mathrm{n}}^2\mathrm{a}}{{\mathrm{V}}^2}$

The pressure of real gases is less than that of ideal gas because the inter-molecular attraction is negligible in ideal gases. Due to the presence of inter molecular attractive forces in real gases, the frequency of collisions of gas molecules with the walls of the container decreases. Due to this, the pressure decreases.