Crowd-sourced ADSB receivers on the ground pick up ADSB transponder signals from aircraft, and relay the data to a website.
Depending on which receiver picked up the signals, reported signal strength would vary.
The ADSB website explains about the crowd-sourced receivers. Maybe you can find a map of their locations. Maybe somebody turned their receiver off or lost their internet connectivity, and the data from a different receiver was substituted in the database.
Think.