From that image it is appears that the left prop was rotating at much slower rate, hence the different motion blur that both props have got on the image - it is 2-2.5 times longer on the right propeller.
If my assessment is correct, it looks like the right propeller was rotating faster - (roughly) 2 2.5 faster, than the left one.
Here - i made a small test to confirm that - the left object rotates at 2.5 times faster rate than the right one.
