This is a very common occurrence during the cooling season. Of course, heat naturally rises, but that’s not the primary culprit. A major contributing factor is the amount of heat that a second floor is gaining from the attic, many attics can reach 150° or higher in the summer. If an attic is under-insulated, this heat has no problem affecting the comfort of the second floor.
Second floors can also suffer from a lack of return air, while the system in the house pulls most of the warm air through the path of least resistance, i.e. the first floor, this results in hot air becoming trapped upstairs. Duct design and thermostat location also play major roles in temperature differences. Adjusting balance dampers can be a way to force more cool air to the second floor.