Hmmm that's odd, I've used API forever for freshwater and saltwater pH. Is it by chance expired or not enough drops? If its filled to the line, 5 drops, and not expired I could only guess it's a bad test or your ph is way too high or low. Has it done this since the beginning?
I'd have to guess a bad test kit, maybe contact API about it. The only way I could think your pH would be any higher or lower then the kit would be if you were altering your params like crazy.