I don't know what I'm doing wrong. Two nights in a row I tested pH and got 7.2. I didn't believe it and immediately tested again using water from the same sample and got 7.6. A third test showed 7.6. I rinsed the tube each time before testing with the sample water. I am careful when I test, (unlike the teenager at the pool store!) How can results be so different from the same bottle of sample water?