how they manage to stay in business
Pool store "advice" and test results are tailored to selling you things you don't actually need.
That's how they manage to stay in business!
Some are more honest than others, but even the best ones are quick to offer up a magic potion of expensive chemicals, supported by their "high tech" test results printout, that offer little to no benefit for your pool. And just as often actually hurt your pool over time (like algicide products with metal in them).
Then there is the problem of testing consistency. How many employees do they have? Are they well trained? How many different ways of testing do all those people have? When you do your own testing, using consistent, repeatable methods, then you can better rely on those test results. Test results reveal numbers on which you can calculate doses correctly. But they also reveal trends over time, which are just as important: "Why am I losing so much FC all the time." or "My CH is rising." Etc. Watching trends help you strategize your overall pool maintenance plan. You can't get trends from inconsistent testing. You can't get consistent testing unless you do it yourself, the same exact way each time.
That's just a couple of reasons. Guaranteed that if you start doubting your own test results and mix and match TFP advice with any others: pool store, pool guy, neighbor, whatever, you'll have trouble maintaining your pool.
I wanted to clarify some advice given by others recently.
Focus on just 3 tests - pH, FC/CC and TA for now. Your CH and CYA are stable unless you add fill water (assuming it has high Calcium) and it will take a while using pucks to add CYA level to record a measurable increase. Remember we measure CYA in units of 10 so you are at 20 now.
That's actually four tests, but who's counting!

Those
are the tests to concentrate on. Adding fill water will cause your CH to rise. That's inevitable and unavoidable if your fill water has CH in it (which it surely does, unless you fill from a water softener). But filling a pool won't affect your CYA level unless you are also losing water by other means than evaporation (like splash out or overflowing or backwashing a filter, etc). Neither CH or CYA evaporate, so they stay in your pool. CYA won't increase unless you add it. It won't decrease significantly unless you
exchange water. It does degrade some over time, so you will be adding some each year, but not much.
If you've determined your target CYA to be 30-35, then don't dose your pool with 30ppm of CYA, as you'll likely overshoot your target. You have
some CYA in your pool, and it may be just shy of what is readable on your CYA test. So it is not zero. It could be 20, as Herman points out. Adding 30 now might put you at 50! If you have kept track of how many pucks you've put in your pool, you can
calculate how much CYA you have. (You can only get away with that with a new pool, because a new pool starts with zero CYA.)
Keep doing what you're doing with the pucks. As Herman pointed out, you've got
some CYA, and it's cold out and your FC is holding just fine. So keep creeping up on your CYA 30 goal and eventually it'll start showing up on your CYA test.
All your other numbers look great.