I know there is a +/- for every test so that could explain the 2/13 test BUT the swing from 12/7 to 2/6 ???? I know some places switch their sources for different reasons. I wonder if that is what is going on with you. That is a wild swing for sure!
What's your water source? Where I used to live (Peoria, IL) the city water supplier drew about 1/4 of the water from the river and 3/4 of the water from wells. I had no need to measure water there but I imagine if you have a similar situation depending on where the water is coming from at the time it could change quite a bit. If the city draws all their water from one source then it seems a little weird.
Thank you for spurring me to talk with my local water company! I learned that they have 2 sources. The main one is groundwater/well, but they sometimes source from the county water district. Looks like the county water is a mix of groundwater and reservoir (both local and imported).
I couldn't find any alkalinity line item in the local company's water quality report, but that report is only annual and 2017 hasn't been posted.
The county reports alkalinity monthly. It's reported as CaC03 and has large swings from month to month: from the ~50ppm range to the ~90ppm range.
I'm not a chemist, but I think there are probably more carbonates than CaC03 in the TA that we measure with our test kits. Is the above variation in county alkalinity the kind of thing that could affect my fill water readings?
Anyway, one thing I'm going to do is start tracking my fill water in my Pool Math App. I've created a new pool called "Fill Water" there and will take fill water readings at least once (probably twice) a month for awhile so I can look for patterns and understand the range that I am dealing with.