Can't really think of a reason really.. Its definetly not "normal" but not out of the ordinary. I see you live in Australia so its possibly hot ( well hotter than the UK anyway ). All I can think is something it making the barometer fluctuate as you say it only goes down not side to site the GPS is good. The phantom takes baseline readings at power on I believe so maybe its detecting changes in pressure,wind,temprature.
Yu dont say the area you are flying in, remote location, urban, city. Try it in differnt locations ( keep in near and low ) and see if it continues to do it.. Also make sure VPS is ON, although at 15 feet its not having much influence.
Remember to buy extra propellers and one or two spare batteries to keep you flying
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