Refractometers, and other measuring equipment for that matter, are only as good as their calibration. If you do not calibrate them regularly, or calibrate with the wrong standard solution, then you could very easily get false high or low readings. When these happen, obviously any adjustments that are made to the aquarium will be wrong. So, calibrate frequently and only use high quality standard solutions to ensure the instruments are working at their absolute best shape. Most solutions are very inexpensive and readily available.