Welcome to TFP!
Pool stores have proven to be incredibly unreliable sources of water testing, second only to test strips. Unfortunately there isn't really a benefit for them to be accurate, so long as the customer trusts them and buys from them that's all it takes. The truth is that most customers will never question them and just assume they are reliable, which is why you asked after all. But once you break it down for what it is, a free service offered to bring customers in to the store and convince them to buy things, you see why it's not something to trust. "You get what you pay for" so to speak.
And really, who has more vested interest in your pool than you? We trust a new person on their first attempt to run the tests more than a pool store because experienced or not they want to get it right. Test results dictate everything that comes after that, good results lead to good care.