Rssi value drops every minute

I noticed some strang WiFi.RSSI() behaviour.
I just read the rssi value every second… Normally, the rssi value is approximately -60 dB

Every minute, this value drops to approximately -80 dB, see also the plot below, of several minutes of recording.

I varied the delay time a little. The strange thing is that the drop to -80 stays no matter how large the delay is.
When delaying every second, every 60th loop shows -80.
When delaying every 2 seconds, every 30th loop shows -80
When delaying every 10 seconds, every 6th loop shows -80
When delaying every 60 seconds, every loop shows -80
When delaying more than every 60 seconds, for example 70 seconds, every loop still shows -80!!

I was wondering if this drop is caused by the cc3300 communicating something (to the cloud or the spark environment)

And I cannot explain that reading the RSSI function shows -80 every minute, no matter what the delay is, even if it larger than 60 seconds! I would expect that with large delays, the rssi value drop has been passed and shows the regular value of 60. Does the current function “remember” the drop?
Any clues?