The height of a given pressure surface above the ground varies with temperature. As an example, consider two identical columns of air (A and B). Since they are identical, the 500 mb surface is found at the same height in each column.
Cooling column A and heating column B changes the height of the 500 mb surface in each column. Since colder air contracts, the height of the 500 mb surface in column A decreases, while in column B, the warm air expands, raising the height of the 500 mb surface.
Therefore, where the temperatures are colder, a given pressure surface will have a lower height than if the same pressure surface was located in warmer air.