I am a retired winemaker and have used pH for years to determine wine total acidity by titrating with a base without fully understanding the chemistry but maybe I can give you the backyard gardeners explanation.
pH is the negative logarithm of the hydrogen ion concentration:
pH = log 1/[H+ = −log [H+]]
We're measuring total hydrogen ions here.
It is not a linear relationship. A solution of pH 6 has 10x more hydrogen ions than a solution of pH 7. A solution of pH 5 has 100x more hydrogen ions than a solution of pH 7.
So you can see that the influence of the water pH in our case has little effect on the pH of acidic solutions in the pH 5 range. I hope this helps. Don't ask me to do the math. I just follow the instructions that come with equipment....