I can think of two reasons:
1). Air scatters and absorbs some light. So the farther the sensor is
from the source, the more light is lost going through air, and the less
accurate the sensor will be.
2). Light SPREADS out as it travels away from any source.
Even from a LASER ... the spreading is very small, but it's there.
This means that the farther the sensor is from the source, the
LESS light will enter the sensor, because the rest of the light will miss it.
More spreading out ===> more of the light misses the sensor.
(That's why, if the lamp is too dim to read what's on the paper, you
move the paper closer to the lamp, where the light is less spread out.
Then, there's more light hitting each square inch of the paper.)
