Today I found out why I got such a high FPS rate 2 weeks ago.
If you open the ATC window and then undock it, it will open again in its old position when you restart the simulator and start a new flight.
I had probably done this and put the ATC window on a second monitor but did not turn it on. So I did not see this window during my tests.
But when this separate ATC window is open, MSI-Afterburner counts the FPS almost twice! I noticed this today after I put the ATC window back on a second monitor.
Two weeks ago I had closed this external ATC window and since then I have not been shown such high FPS values. Until today!
So this mystery is solved. 