Hi @bskim, welcome to the forum 
This thread is the most relevant discussion of the confidence values of our 1-dimensional series of ping sonars. I’d suggest reading through that, and some of the links from there, to get a better understanding of what the confidence conveys and what goes into the distance estimation process.
I would not say that’s a common experience, but it’s at least theoretically possible if
- the motion of the ship is pushing aside vegetation and/or scaring away marine life that then go into the beam path when the ship stops
- the ship’s momentum is reducing pitching and/or rolling, and keeping the beam pointing at a more consistent area of the surface
- this is mostly relevant if the water is quite rough, with significant rotation when it is “stopped”
- moving through the water is damping the sonar’s transducer vibrations, and allowing it to detect shorter distances than normal
- this seems unlikely, and would only be relevant if the bottom is very close to the sonar
- the ship’s motion is temporarily solidifying a soft river bottom, so it more effectively reflects the sonar pulses
- likely some other scenarios I haven’t thought of
Is the behaviour consistent? And have you tested the sonar in other environments, to see whether you’ve had similar issues?