This is not unexpected and is most probably due to the erratic weather conditions of the ionosphere.
The effect may be likened to the reception of short wave radio signals.
These fluctuations in level would also account for the many reports from sufferers that the Hum appears to be switched on and off, this effect would become prominent as the levels reached the sensitivity threshold of the individual.
The chart above left show a sample of the fluctuations and was taken from Friday March 20 at 3 pm through to Saturday March 21 at 11pm. Readings were taken at approximately one hour intervals. Readings were made at Bristol UK. at a time of high atmospheric pressure with clear skies and low winds.
The graph lower left was made on March 27 at a time of low atmospheric pressure with high winds and rain.
The lower graph shows a fall in energy level of 25 per cent which is roughly in line with the observations of Hum sufferers.