The low pitched whistles do better for two reasons. First is age induced hearing loss starts with the higher frequencies, so older people will have a lower threshold for detecting low pitched whistles then high pitched whistles.

The second is acoustical absorption is an exponential function. Roughly put, every time the frequency doubles, the absorption into the air increases by a factor of 2.7. So, a 2 KHz marine whistle will have 2.7 times the sound pressure as a 4 KHz referee's whistle at a given distance with all else being equal. Wikipedia has an adequate explanation if you want the math.

en.m.wikipedia.org/wiki/Acoustic_attenuation