Wien Bridge Oscillator - LT Super Low Distortion Variable Sine Wave Oscillator Debug
It is the task of the LT1055 opamp to (nearly) act as peak rectifier. However, because the rectified voltage must be able to change in both directions (control function) there must be the possibility for (slowly) decreasing of the peak value (discharging of the feedback cap). That is the function of the 22k parallel resistor. Hence, the whole rectifying circuit has a fixed time constant which must be at least 10 times the oscillation period (rule of thumb). 

As a consequence, the time for discharging the feedback capacitor depends on the oscillation period (when the diode is off). That means: The rectified mean voltage which controls the FET resistance - and, thus, the oscillation condition (and the oscillation amplitude) - is NOT independent on frequency. 

With other words: To meet the oscillation condition (opamp gain of "3") the FET resistance must always have the same value. Hence, the rectified voltage must have the same fixed value (control voltage). But this voltage depends somewhat on the oscillation period (more time for discharging the cap for lower frequencies). Thus, the control voltage lowers for smaller frequencies. This "drift" of the rectified voltage is automatically compensated by a larger oscillation amplitude.