I became a BBB user since July 1, and use 3 lb shocks and 2 tablet during past two months. the CYA level tested by a pool store was 100 ppm on July 4. my CYA test result using TF-100 was 40-50 on July 8. The CYA level keeps the same range of 40-50 using TF-100. I went to the pool store to purchase MA and let them test water, the CYA level became 28!!. I tested two days ago, it was still 40-50.

I think my test result is more reasonable since there is no rains during these days. Why their testing results changed that a lot?

Trust your own results. There is no way CYA could have gone down as much as they said unless there was massive water replacement.

The black dot test in the TF100 is the best one available short of rather more expensive laboratory grade tests. Pool store testing is sadly amazingly unreliable. There are some good stores out there, but there are also many bad one. The details of why their testing is bad vary depending on what kind of test kit they are using. Some stores use test strips which are especially unreliable for testing CYA. Others simply do the test wrong, even when they are using a good quality test kit.

I think a good rule of thumb is when you own a good test kit such as the TF-100 or Taylor K-2006 is to always trust your own results and never take a sample to a pool store unless maybe you suspect metals because it always makes people second guess there self even though most pool stores results have been proven to be notoriously wrong.