I checked the Panasonic Manual.  The manual says that you can have the shutter speed set as high as 1/2000 with the flash BUT if you have an external flash, it will set it between 1/30 and 1/250.  So my guess is the camera will automatically set the flash shutter speed to 1/250 even though it displays 1/2000 when you use the built-in flash (idiot proof design).  So if 1/250 is the fastest flash shutter speed, then at 1/500 only half the image is being exposed at a time, 1/1000 would be one quarter, 1/2000 would be one eighth (a very thin slit passing across the image sensor).  Base on the distortion in the propeller photo, it looks like your shutter moves top to bottom in the camera, which is bottom to top on the image.

 

Mike Easley