Because the Micropulse monitor is notoriously untrustworthy. Mine didn't show the led for the blank tank for months and then it worked again - well, worked is a relative term.
Do a search on "Micropulse" and "seelevel" and you'll get tons of good info.
In the meantime, you can either re-calibrate the sensors per the manual, live with it (so long as you have a sense of how your usage impacts the levels of the tanks) or swap out for the seelevel system (or some alternative).
If yours is still under warranty, start with the dealer and have them recalibrate.