No. I do not. Reducing of 50% of your CYA would require replacing 50% of your water WITHOUT any mixing. If it was rain, you were very slightly diluting the CYA, then draining some off by backwashing. Pool stores are typically very bad at testing for CYA. Your result I would trust. To get more comfortable with your own test, make sure that you fill the tube and observe the result. Then pour the solution back into the dispenser bottle and fill the tube again. See if your observing a consistent number. You can do this multiple times with the same sample. It's also important to perform the test in full sun, with the sun behind you, holding the tube at about waist height, looking down into the tube.