Capt could it possibly be the larger size vessel, along with co2 and water temp fluctuations? You say you run a pump in the res but nothing in the 1 gal jugs.
I read this here and it makes sense:
Rising pH in the Reservoir
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CO2 dissolves in tap water, in often copious amounts, due to bubbling and frothing in the plumbing. It comes out quite cold, usually at around 50°F. As it slowly warms, carbonic acid turns into carbon dioxide which evaporates along with other gases.
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From what I'm gathering, your tap water is a certain temp, and it stays relatively constant in the jug. Your larger reservoir along with the pump warming the water causes carbonic acid to turn in to carbon dioxide, evaporating off, essentially decreasing the amount of acid in the water causing pH to rise. The water pump running keeps this cycle going.
Experiment. Pull the water pump and put in some fresh water. Let it sit for an hour or so, then test pH. Repeat at your typical frequencies. See if the pH rises at the same rate.
FWIW, my 250 PPM tap starts in the low 7s then breaks 8s steadily over the course of a week. It's warm in my grow area so the heat contributes to this reaction.