Real gases sometimes display less then ideal behavior. The Kinetic Molecular Theory of Gases has two major flaws:

First, real gas particles are attracted to each other. As a result, real gas particles draw together tightly making the actual volume less than expected. (This is an attractive interaction.)

At pressures greater than 300 atm, the actual volume of most real gases is smaller than predicted based on the Ideal Gas Law. However, at low pressures, the assumption still holds because the gas particles are relatively far apart.