It's not a lot, but I suspect it has something to do with rounding. For example, my Zone 9 ran for 4 cycles @ 45 minutes each, putting down 0.25 in/hr with 70% efficiency. If the Iro looked at them in aggregate, it should calculate 3 hrs * 0.25 in/hr * 70% = 0.525 inches (0.53 with rounding).
Instead, it shows 0.48 inches. I know that's only an 8% difference or so, but why should there be one at all? I'm just guessing here, but I'll bet the Iro is calculating each cycle separately and rounding each calculation down to 2 digits, adding at the end. In this case, it would be 45 minutes * 0.25 in/hr = 0.188 inches, rounded down to 0.18 inches. Then times 70% efficiency = .126 inches, rounded down to .12 inches. Then multiplied by 4 cycles = 0.48 inches.
Any chance that is what's happening? Over the course of the year where we spend $ thousands on water, those small differences could add up? In any case, it would be helpful to understand if the calculations could be made more precise.