The MC95R sensor provides a strong response from Extremely Low Frequency (ELF) through Very Low Frequency (VLF) ranges, with a very strong maximum at resonance near 1700 Hz. Its frequency response is the same as MC95RW in the graph. The resonant frequency (usually near 1700 Hz) may vary considerably due to individual 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.