Water balance problem

I have been using W2 for a simple reservoir model (1 inflow and 3 outflows). In a spreadsheet, I have calculated the reservoir storage through time using the initial storage, the inflow, and the outflows. In this calculation and in the W2 model, evaporation and precipitation are not included in the water balance. Through the year, the modeled reservoir storage deviates a little from the the spreadsheet calculations. I have eliminated step violations. I am wondering if this problem has occurred before or whether I should be looking harder for an error in my assessment.