There are so many factors that come into play during a water test. You have mentioned some things that may be the reason. Your logic is solid and Its more likely you've already addressed the culprit but I wanted to hit a few things that were not expressly spoken of. . .
The alkalinity should not be discounted: Keeping it in range will allow the pH to stay balanced as alkalinity will absorb the things that will potentially want to draw the pH higher or lower. I prefer to have that in line first usually, then balance the pH.
Additionally- I have found that usage patterns (how many users, how long is each usage, and how often?) can reduce pH. If you are using your tub with great frequency or with many bathers or multiple times a day, it may have an effect on your pH.
Your source water (tap water? well water?) could also adjust your pH level. Thus every time you top off your spa you may be making adjustments. Test your source water and know that its levels will be constant and will push your spa water in the direction in which they read.
Chlorine is typically high in pH. But if you're maintaining a lower chlorine level you wont get the pH bump. Liquid chlorines (bleach, liquid shock, etc) typically has a shelf life and the more it is jostled around, the more gas it gives off and the lesser the potency of the product.
And your testing kit could be contaminated. The Taylor kit is awesome and I dont knock you for having one. But what if the liquids have gone bad? Or- when was the last time you've had the water tested by a professional just to get a second set of eyes and a second opinion on your water?
I am only pointing these things out as you didn't address them specifically so I wanted to make sure I wasn't missing anything.
As an aside: Testing every day can also be the issue. You can test your water three times a day and get three slightly different answers. There are margins of error, slight variations in water chemistry and user error that can contribute to the variations. I'd push back to 2-3 times a week so you can see the bigger picture. And give your spa several hours to adjust between, say, making a pH adjustment and adding chlorine. Then test several hours after the final adjustment is made.
I hope this helps. I am sure you probably knew all of this already but I just prefer to make sure all bases are covered and that nobody is missing something easy. Good luck moving forward.