Unexpected Effects from Different Generic Medications

As a fitness coach focused on optimizing physical performance, I've noticed something puzzling with my own medication. I take 25mg atenolol for blood pressure management, but I've experienced very different effects from different generic versions. The Unichem brand causes severe bradycardia and chest discomfort, seriously impacting my ability to exercise and coach effectively. However, the Teva brand works perfectly - I feel great and can perform at my peak. This difference seems counterintuitive for identical dosages of the same medication. Has anyone else experienced this? I'm curious to understand the science behind why generic versions can vary so much in their effects. This knowledge could be valuable in helping my clients optimize their health and fitness journeys.