evaporation .... like takagari said. you drip in 40 gallons per day, but you evaporate out like 30 gallons a day.....
but, I, on the other hand, have been thinking about it longer and have nothing better to do....so I cover my sump and I evaporate only 2 gallons per day, while I still drip the same 40 gallons per day....
so...if ya don't care how much tap water you use up....just drip. measure your output, if that doesn't have any chlorine/chloramine, don't worry.
BUT...if you DO care about wasting water, then cut evaporation by covering your sump. You'll find that a covered sump lets you wait much longer until your fish outgrow your drip.