Not quite sure how the GH and KH could be fluctuating so much in the water supply, but aside from why, the indicated levels are in the "low" range which means soft water. The less mineral in the water, the easier it is for the pH to fluctuate.
In an established aquarium, the pH naturally tends to fall as the water acidifies due to the ongoing biological processes that produce carbonic acid that lowers the pH. In hard or very hard water with a high KH (carbonate hardness, a measurement of the bicarbonates) the carbonates and minerals bind with the acids to prevent the acidifying of the water. We term this "buffering" the pH. In very soft water, there are fewer minerals and carbonates so the carbonic acid remains and the pH falls. Plants also impact this, as does introducing CO2 by diffusion, though these two processes work oppositely. Plants through photosynthesis consume CO2 so the water pH rises during the daylight hours, then falls during darkness, what we term the diurnal cycle which is harmless and common in nature and all planted aquaria. Adding CO2 lowers the pH because it produces carbonic acid which lowers pH as explained above, and continuing to add CO2 will exhaust the buffering capacity of the hardness; this can cause a sudden drop in Ph , termed a pH crash, which can be lethal. This is why CO2 should only be added to planted aquaria (so the plants can use the CO2) and never at night.
Regular (weekly) partial water changes of 50-60% will usually work to maintain stability, especially with softer water. However, this has to be balanced. There is absolutely nothing wrong with a pH around 6 if you have soft water fish; most plants occur in acidic soft waters as well, so they will be happy. The pH should be regularly monitored to determine the specific reaction of biological actions and water changes; you don't want sudden fluctuations. I would not do massive water changes because it will significantly raise the pH that will then fall again. My tap water is around 7 or 7.2, with basically no hardness; my tanks run around 6-6.4 and when I do the 50% water change the pH only rises by .3 or .4 which is insignificant to the fish. Water at pH 8.2 will result in a far greater change each time, and that must be controlled.