Its not just the water, it's also the
nutrients that you add to the water that over time that can react to whatever is in the water, and can continue to change the pH. Also the accumulation or build up of the minerals like calcium and lime in the system, that over the weeks that could make things worse. I don't check my pH daily of my water supply, but mine seems to change from time to time, at least the RO water. I figure this because I check the pH at every
nutrient change, using the same amount of water and same amount of the same nutrients. Mostly I need to add a little pH up (lately anyway), but from time to time I need to add pH down instead. I never know witch it's going to be, so I check it first before adding either up or down (only after adding the nutrients of coarse).
I started out checking the pH every day to check for changes in my systems. Over time I got lazy with doing that, but if I add any significant amount of replacement water I need to check it again. I do try to check it every 3 to 4 days though, over time you can usually guess when it's likely to be out of range. I think you are doing good with your testing plan, that makes scene to me, though again that is just the water, and not taking into consideration any possible reactions with the nutrients. But I might also try retesting both samples every day, over say a weeks time and make note any changes.
I think your best bet is to use pH adjusters made for hydroponics as you said you were probably going to try. Though from here I have no idea what's in your water supply, so it's hard to give any advice there other than some sort of filtered water would be better than straight tap. I use RO because it was already installed in the house when we moved in, so I already had it to work with. Although I am looking into other options because the current system only puts out about a gallon per hour of RO. For the systems I have in mind for the future, that simply wont do.