Why you use minus to zero range in drum level transmitter?

I refer to Steam Boiler drum level.

It is actually - to some positive figure is the range. Generally the boiler level is maintained at mid point that is zero. The transmistter is differential type. (DP type) If the transmitter is with a specified signal out put of 4-20 mA, it sends out 4 mA at lowest level and 20 mA at highest level. At mid level zero it sends out 12 mA. Differential pressure is highest at actual lowest level and lowest at actual highest level of the drum.

By keeping negative to positive range there is fair amount of analog signal on lower and higher side of the drum.