Both the signal and the noise vary at any instant in time, although they are presumably constant in the average. International Light radiometers integrate even in signal mode, averaging over a 0.5 second sampling period to provide a significant improvement in signal to noise ratio. The ability to subtract ambient light and noise from readings is a necessary feature for any radiometer. Even in the darkest room, electrical dark current in the photodiode must be subtracted. Most radiometers offer a Zero button that samples the ambient scatter and electrical noise, subtracting it from subsequent readings.
Integrated readings require ambient subtraction as well.
In flash measurements especially, the total power of the DC ambient could be higher than the power from an actual flash. An integrated zero helps to overcome this signal to noise dilemma.