The MC95R sensor provides a strong response from Extremely Low Frequency (ELF) through Very Low Frequency (VLF) ranges, with a sharp maximum at resonance near 1.5 kHz (for cylindrical version of MC95R) or near 1 kHz (for rectangular box with panel BNC). The rectangular version also has a lower peak output voltage at resonance. The resonant frequency may vary considerably due to assembly variations and the complex output impedance the sensor output is connected to. So the calibration curve (Vo/field) may differ from your results due to different output impedances at resonance and higher frequencies.