The maximum amount of solar energy available at any given location in the world varies by time of year (i.e. season), latitude, time of day, cloud cover or haze, and ambient air temperature. If you're not getting the full 100W, you may not have optimal sun conditions - for example, during the late afternoon in the Spring, Fall, or Winter.
The peak rated power is given at standard temperature and a standard solar irradiance value. Power output will be lower if the irradiance is low for any reason.