location of pingers
The uncertainty comes from : the sub's navigation uncertainty added to the bias on sound propoagation. Considering that reckoning the actual sound velocity profile in such environmental conditions proves rather difficult, the expected detection range will greatly vary. therefore, when eventually you have something on your sonars, you never know whether your are at the limit detection range or whether you just happened to cross a specific sound ray. Most of the time, you'll assume that you were at the limit detection range, which increases the uncertainty.
Also, if you want your position record to match your recordings, in order to be able to determine where you were sailing when the detection occured, you need a very accurate time measurment on both logs. This adds too to the final estimation.