Some use nitric acid for veg, which is N, and some use phosphoric acid for flowering, which is P. They won't change your PPM reading, but they obviously are changing your feeding ratios. You could try compensating for it, but I've yet to see anyone do that. It's probably not worth the stress or effort. Plants still grow... etc.
Gh PH down is not that great though, I don't think. It uses a variety of acids, which may even do more to keep the ratios in check. But the citric acid in it doesn't help stability and bacteria can feed on it causing a bloom.
Anyway, once you establish your res with all the ratios you were hoping for, it doesn't stay that way. The plants will start taking what it wants from it. It would be interesting to see some real data on what the expected variance should be under standard and even different conditions. Someone, like a professional botanist, probably knows, but I've never seen that info.
I would chalk it up to "not a big problem" and push on. Just worry about keeping your res healthy (keeping your roots healthy), and just make sure that your PPM's drop over time rather than rise. That's all you need to know that your plants are content.
You could have everything perfect, with your PPM's dropping, and then pull a few big leaves off and the next thing you know they're rising. Point is it's constantly variable and you can never know what your plants are using exactly at any given time, other than some portions of something you gave them. Just monitor and know what to look for.