Strobe Utility sometimes has problems when your BenQ/Zowie monitor is not the primary.
This is the same problem as trying to run http://www.testufo.com
simultaneously on two different-Hz multimonitor: It just doesn't work properly.
The frame rate usually synchronizes to the primary monitor, but sometimes it synchronizes to the wrong monitor.
Try minimizing all windows except Strobe Utility, then try again. (Make sure the Strobe Utility adjuster is on the same monitor being adjusted too -- e.g. no window on the 100Hz)