Test Panel / holding tank

I ran some test today on the grey water storage, since I feel like its filling up much faster than it should. I calculated that my holding grey water tank only takes 3.5 gallons for the test panel to show it as full. Reading today that the holding tank should be 30 gallons, I am rather confused.

Any ideas what I can do to fix this? or any other test I can try to find out why this is testing wrong?