Some general things to recognize from this data. First, there is no scientifically-controlled meaning to the subjective terms like soft, hard, moderate. This is like using common names for a fish species, it only really makes sense to the person using it because he/she knows what they mean but most of the rest of us do not know what they mean.
For example, it says Oklahoma water is hard. Then it says the average is 146 ppm. This is not by any stretch of the imagination hard water, at least not to an aquarist. A GH of 146ppm is roughly 8 dH, which the hobby generally terms moderately soft. Soft water species will be more comfortable in this water than any true hard water species; this number is generally speaking too low for livebearers, rift lake cichlids, and similar.
This is why we always say, get the numbers, not a term that can be very misleading.
This settles the GH question, giving us an idea of suitable fish. There still remains the pH though, and given that it readily falls from 7.6 to 6 in an aquarium with no fish, I think they are likely adding something to increase the pH. This is worth checking, through the water authority. However, another explanation is that the KH is low, lower than the GH, and this buffering capability is therefore minimal and the pH naturally lowers. Though that usually occurs in an aquarium with fish producing organics.