At high pressures molecules are colliding more often. This allows repulsive forces between molecules to have a noticeable effect, making the molar volume of the real gas greater than the molar volume of the corresponding ideal gas, which causes Z=RTPV to exceed one. When strong repulsive forces among the molecules of the gas dominate, making Z>1. The closer the gas is to its critical point or its boiling point, the more Z deviates from the ideal case.