These differences are caused primarily by two factors. First, the Earth's surface is not composed of uniform materials. The two surface materials that dominate are water and land. These two materials behave differently in terms of heating and cooling causing latitudinal pressure zones to be less uniform. The second factor influencing actual circulation patterns is elevation.
Elevation tends to cause pressure centers to become intensified when altitude is increased.
This is especially true of high pressure systems. Atmosphere pressure values are adjusted for elevation and are described relative to sea-level. The slider at the bottom of the image allows you change the time of month. Blue shades indicate pressure lower than the global average, while yellow to orange shades are higher than average measurements. Wind direction is shown with arrows.