That's a normal variation due to load changes on the power grid.
Years ago when I lived in Columbus, GA and worked at WRBL-TV, our transmitter plant shared a sub-station with the Sunshine Bakery Company, which had electric ovens, and 2 other stations co-located on our tower. Every afternoon when they started firing up the ovens, we would have to adjust our transmitter power output to compensate for the line voltage drop.
The 4 of us were the only loads on that sub-station, but even with a full power VHF, full power UHF, and Full power FM transmitters, we were considered the minor load, so this happened almost every day of the week. Our contract with the power company specified that no other customers would be put on that sub-station to minimize further line voltage swings..
It was a huge sub-station. The bakery was, as the power company engineer told me, one "God awful load" on their system.
But, it sure smelled good around town when they were baking cakes and cookies!
Dave