As Franktafl says, what was your filling protocol?
I'll assume you started with a dry system and first filled fresh water until full before starting your measurements at the sink.
Is there a water heater tank? If so, maybe 6 or 10 gallons of fresh transferred to that tank from the fresh tank upon first filling.
Did you also collect water from low point drains to account for the volume of water in the plumbing?
2012 Aspen Trail 2710BH | 470 watts of solar on the roof | 2x6V GC batteries | 1500 watt PSW inverter | Micro Air on A/C | so far strictly boondocking