In all honesty, my guess would be that either A) you've got a small leak. Not much, but a little bit. B) it's not airtight and some of the water is evaporating. C) there is still a bit of air in the system and it's working it's way out. Sometimes tiny air bubbles will stick to the metal, even with decent flow. These air bubbles could eventually be washing out to get caught in the airtrap.
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