Best guess, sunset is being defined as a specific time. Sunset isn't necessarily the point at which all sunlight is gone, and even then, the time the sun goes down varies depending on where you are within a given time zone. If they just used an arbitrary time or an almanac listing for twilight for each day, they could easily end up including slivers of daylight in their post sunset data.

Which is really pointless since this is just the same old "BuT sOlAr DoEsN't WoRk At NiGhT" argument that idiots have been making for decades. But I suppose you gotta at least pretend to use real world data to make it look sciencey.