The wattage rating of a guitar amplifier refers to the power output of the amplifier head, not the cabinet. Therefore, in your example, if you have a 10W amplifier head and connect it to a 100W cabinet, the maximum volume you will achieve is still limited by the 10W output of the amplifier head.
The wattage rating of the cabinet indicates its power-handling capability, which means it can handle up to 100W of power without being damaged. However, if you connect a 10W amplifier head to a 100W cabinet, the cabinet will not make the sound louder than what the amplifier head is capable of producing. The cabinet's higher wattage rating simply allows it to handle higher-powered amplifier heads without being overloaded.
It's worth noting that the wattage rating alone doesn't solely determine the loudness of the sound. Other factors, such as the efficiency of the speakers in the cabinet and the overall design of the amplifier and cabinet, also play a role in determining the volume level.