My guess is that the water doesn't move much faster than 10 m/s which translates to 1 cm/ms. You certainly can't make out 1 cm differences in the images, so the

My guess is that the water doesn't move much faster than 10 m/s which translates to 1 cm/ms. You certainly can't make out 1 cm differences in the images, so the "same millisecond" claim is definitely exaggerated. From a quick look, the time difference could even be more than 10 ms, but not by much.