Question: How are you measuring the frequency?
If the output waveform is something other than a nice, low-distortion sine wave, your frequency counter may be reading something other than the actual (fundamental) frequency of the sine wave.
If you have a dual-trace scope with probes, do a differential measurement of the output and measure the frequency using that.
If you don't have that - or don't want to mess with setting it up, measure the frequency again using a 100k resistor in series and a 0.1uF capacitor (of suitable voltage rating!) across the frequency counter leads as a quick-and-dirty low-pass filter to see if you get the same results.