I understand and I was surprised as well. The only thing that makes sense to me is that Im on a well. My water must be over saturated with what ever is buffering the PH up - calcium carbonate or what ever. Adding some PH down, will fairly quickly use up what ever is fully dissolved and chemically available ‘in solution’, but there is still more that is not fully dissolved and ‘available’. Whats left is suspended in the solution as very small, undissolved particles. Its those undissolved bits that will continue to dissolve when the saturation level drops after adding the acid. These are the kinds of things that get filtered out by an RO system.
From what I have recently learned, using RO water can cause other PH swing issues precisely because there are no buffering agents in the water - making it much easier to swing the PH either way with just small additions of acid or base materials…