Why doesn't the cooling fan system operate immediately even though the cover of the computer has already become hot?

The cooling fan system does not operate even when the cover of the computer becomes hot after the VAIO computer performs some tasks.

PCG-GR* has been designed to dissipate heat through the cover of the PC. After some duration of operating the computer, the bottom layer of the computer gets hotter. Please use the tilt stand when operation the computer on a flat surface. Sometimes, it may be very difficult to hear the operating of the cooling fan system.

User can find the same explanatory note bundled with the system.