A load test is the standard that these stores use. If the battery passes the load test, which means it can deliver the CCA that the battery is rated for, chances are it is not the battery, and they don’t feel obligated to replace it.
The next time you experience a slow start, don’t let the engine start up. Take the battery out and use another car to take it to the store for an immediate load test. It may be a matter of letting the alternator charge up the battery that helps cover the bad condition.
A bad battery case: I had a client come in with a fried alternator in a Ford Explorer. We replaced the alternator, but noticed the new alternator had a strange hum. But, it tested fine, and we sent him on his way. A couple of hours later, he called with the truck not able to start. It jumped fine, but the alternator was fried again. We put a warranty replacement alternator in the truck, and it too had that strange hum. My ammeter told me it was pulling over 95 amps. We shut it all down and checked everything we could. I pulled the battery, and took the client to the store he purchased it from. It checked out voltage-wise, but I had them load test it anyways. The electronic tester they used called for a 30 min charge before it would test it. We waited. Once it ran the load test, the results came back 70% capacity. They replaced it under warranty. Once the new battery was in, the alternator no longer had the hum and the amp draw was a respectable 25-30 amps. My best guess is a bizarre short in the battery that was killing alternators. If that battery had managed to pass the load test, they would not have honored a warranty replacement.