Pool store testing is notoriously unreliable, because the person doing the testing in general has not been adequately trained. Things they do that shouldn't be done include not rinsing equipment between customers, not using the drops right, not calibrating machines daily, and so much more. Finally, they don't really care about your pool. We here at TFP don't put much stock in pool store results; so much so that we don't believe them at all.
You, in the other hand, have a vested interest in making sure your tests are accurate. You also will be trained by Pool School, our videos, and manufacturer's videos. Plus, you have a forum full of people ready to help with any tests you're having trouble with.
Not to mention you need to use an FAS-DPD kit to test chlorine during a SLAM. Pool stores usually use the OTO test, which only goes up to 5 ppm FC.
A test kit is your best bet to getting your pool clear.