Most of the standard zappers can be off by as much as 15 percent. There are better more accurate ones such as the CC1 with as close as 0.25 percent that also provide many different frequnecies, providing a better chance of hitting the target.
A 30 kHz zapper might miss the specific frequency by as much as 30,000 Hz! A 2.5 kHz will get much closer. The trade off is that lower frequencies do not produce as strong of a harmonic at the target frequency.
A big question is how accurate and reliable is your frequency meter? Those found on Digital Multimeters can be way off because they are made for sine waves.