I used to fly airplanes for a living, and there were tmes when the jet misbehaved in the air, we'd land and write up the problem, and the ground crews couldn't find any faults: a CND (Could Not Duplicate). We'd go fly the plane again, and start the process all over again. Often this went on for a while, repeating several times. At some point, a maintenance expert on the problem system would fly along with us, and the fault would present itself in the air; then it would get fixed or replaced and sent to the backshop or the next higher level maintenance facility.
If the service facility doesn't take the time to really exercise the camera, which it definitely should, they might not really get the item fixed. I'm guessing that there is a lot of pressure on Nikon's service personnel to minimize the time a system stays in the service center, and to maximize the number of "service successes" that they have. I'm guessing that this is the case because I'm seeing "it" elsewhere. "It" being the numbers are more important than the method. Many of us are seeing the results of this throughout our daily lives.