Why do the fan blades change shape?

Monitors don't simply flash on and off. In fact, almost all of your screen is dark almost all of the time.

The monitor paints pictures using just one horizontal line at a time, starting with the top line and quickly scanning to the bottom. You only see a bit of the fan blade silhouetted in front of the moving line.

The fan blades on one side are going "down" in the same direction as the scan lines, so they stay in front of the flickering line o'light longer. Result? A stretched out, wider shadow.

The blades on the other side are moving "up", in the opposite direction of the scan lines. They spend way less time in front of the flickering line, so they have a chopped-off, skinny look.