If they are looking at the 'Pings', then in free space the strength of the ping will attenuate as the cube of the distance... However if some form of layered ducting had taken place, then the attenuation would have been in the order of the square of the distance.
Also as anyone who has first-hand experience with radio wave propagation will testify, it is possible for enhanced ducting to have taken place, where the signal is beamed an even greater distance still, where the attenuation is just directly proportional to the distance involved.
Different temperature layers in the ocean could act as reflector of the sound waves. Much the same as a graded fibre optic strand keeps the light signal within the core of a fibre optic cable. Under some conditions the Pings could have travelled much further than first thought.