The reason it drops over time is because the cooling system will slowly ramp up the fans in the laptop to blow more air, thereby increasing the cooling ability of the laptop, and cause the CPU to cool down. What you're seeing is when you start a game or benchmark or something CPU intensive, you see the CPU heat up very quickly, which is normal even in desktop systems.
The fans don't move as much air, so less cooling capacity. The heatsink and fans are smaller, so there's less area to dissipate the heat, much less remove it from the system. More of less, your CPU can heat up VERY quickly, and has very little cooling capacity compared to a desktop system.
