It simply looks to me like your original hypothesis was mostly correct. Based on your data, at whichever depth you do the test, the ADCAP seems to detect you around 1700 yards. To me, this is expected behavior.
Sound speed has no effect on the quality of an active return, to my knowledge.
Occasionally some factors may favor the ADCAP, to pick you up beyond a mile, some factors less than 1000 yards. Maybe there are some random elements to the sensors active capability in game?

What "Lucky" vs adverse sonar conditions, other than the speed of sound through the water? I couldn't say.