Greetings of the day,
The mask’ area should be equal to the number of pixels in the mask times the spatial resolution of the sensor but why it is not the same
number of pixels = 1575781
so the area should be 1575781 X 5m X20 m which is 157.578 sq km but the mask area is 146.459 sq km. why?

After terrain correction, your pixels are no longer 5x20 meters. They will be resampled to squared pixels during this process with a size that is given in the operator