I think this is a fascinating discussion, but since it's getting technical, I'm going to move it to the China Shop.

I generally add about a gallon of regular bleach every two days--to 19,200 gallons. We've had a lot of rain lately (unless you are a hermit you know that NJ and NY have been HAMMERED with rain) and that tends to be acid.

OK: so before we get to the esoteric let's look at some obvious factors:
1) Amount of bleach to pool volume. I add 1 gallon to 19,200 every other day...dilution alone explains a lot of it.
2)Powders dissolve rather than dilute--is that a factor?
3) Total Alkalinity---let's remember this is SUPPOSED to keep your pH from moving. Usually, mine is in the safe-for-vinyl/unsafe-for-concrete range 140-160. This year I'm only running 80-90 (too lazy to add baking soda) and the pH has moved more than it does, but not so much that I'm concerned. I don't care if pH moves between 7.3 and 7.6 and down again as long as it's in that range.
4) Acid rain. Doesn't take much to lower pH.

That's all I can think of this morning....