Water chemistry change, moving to a new house

My girlfriend and I found the most incredible house out in the country. We will be moving within the month if everything goes as planned. So I went out today and got a water sample to test it and I'm a bit surprised with the results. I did tests with both my test strips and my test kit, here is what I came up with:

PH strips: <6.2 - kit: 5.0
GH strips: 25-75ppm - kit 3.0
KH strips: 80ppm - kit 5.0

Ammonia, nitrite and nitrate were all zero. The house has been empty for a month, so the water hasn't been run in awhile. When we first started it up it was dirty and brown looking (probably rust in the pipes?), so we let it run for awhile until it was clear before getting some water to test.

The first test I did was PH and when it was so low, I really expected it to be extremely soft with zero buffer (KH). I'm happy that it does have some buffer to it, but still concerned about the PH. I'm guessing (hoping) that since there is decent GH/KH in the water, there is probably a reason the PH is so low like perhaps because it's been sitting unused for so long or maybe because it just rained a lot the past few days?

I'll be testing it again in a few days to see if anything has changed, but still wanted feedback here on what could be causing it to be so low.

I left some sitting out all night and retested this morning. My pH is in the 7.8-8.0 range, so that's more what I expected with the GH/KH where it is. I read the CO2 chart that comes with my kit and my CO2 was at 1581 with the original test results LOL

Thanks for the reply happychem. I knew something was wrong and it would likely correct itself, but wanted to try to understand what was happening with the water to make it like that