how can air pressure at the same altitude be different?
It all has to do with the uneven heating of the Earth's surface and the global wind patterns. Basically, hot air rises and cold air sinks. As this air cycles in a transfer of energy called a convection cell, it is also influenced and affected by global wind patterns. This in turn creates high and low pressure systems. Low pressure systems are related to storms. Think about this, if an area is under low pressure, then air is allowed to rise up into the atmosphere whereas in a high pressure system, it is being pushed down to the Earth's surface. Because of uneven heating, we are able to have different pressure systems at same altitudes.