I just finished my first grow in soil and while it started out with a pH of 6.7, at some point it shot up to 7.2. I pH my water to 6.5-6.7. Not sure why, but a flush would not bring it down. How would you lower your soil pH if your already into flowering?
Your pH likely rose because your plant utilizes predominantly acidic nutrients. It doesn't matter if your plant is flowering, there are various ways to lower the pH. Depending on your specific circumstance this could be as easy as fertilizing your plant, without adjusting the pH of the fertigation solution. If you're using a bunch of pH Up all of the time to raise the pH of your water when you add nutrient, this could be a contributing factor.
As others alluded to, it has a lot to do with medium. If you're growing hydroponically; in rockwool, DWC, NFT, etc. pH is extremely important. I might not be capable of explaining this in excellent terms at the moment, but consider when you are growing in rockwool or nutrient film technique (where the roots essentially hang straight into nutrient-rich water); there is little or nothing to 'buffer' the nutrients or help regulate the pH naturally. You don't want your roots hanging directly in strong acids, they could burn for one thing and there is also nothing to help plants access those nutrients that would otherwise be locked up in this condition.
Which brings another point, as you mentioned that rain water is a bit more than slightly acidic. But, rain falls on the ground where there is soil. The rain having a pH of 5.6 or whatever doesn't mean that the soil shares the same pH, because there are a lot of things in soil that affect pH and basically just a hell of a lot more going on there.
Many organic/soil growers are not so concerned about pH, and they really don't have to be. For one thing they use amendments that help to stabilize pH in the long term (oyster shell, dolomite lime, etc.), but beyond this those who grow organically in soil are aiming for the soil in their containers to support a complex array of life. Just like soil outdoors, we want beneficial fungus (mycorrhizae, trichoderma), bacteria, protozoa, even earthworms and various insects in our soil because they do many wonderful things for our plants, not the least of which is keeping the pH around where we want it.
Disinfectants in tap water (some which do not dissipate), hydrogen peroxide, synthetic fertilizers and chelating agents will either kill the microherd entirely or just leave them severely diminished and unhappy. This is why these things are avoided.
