I guess the obvious first question is whether or not the starboard engine is actually running 400 rpm too low. You should check it with a handheld digital tachometer to know if there is actually a difference between the indicated and actual engine speed.
I'm pretty sure the Volvo TAMD41 diesels all use a tachometer sensor p/n 862251 which is located on the timing gear casing. Here is a parts breakdown
for your reference. If so equipped, make sure the sensor wiring is clean and tight. You might also make sure the sensor is clean and well seated. As the gear teeth pass the coil in the sensor, a pulse is created that triggers a counting circuit in the tachometers.
If there is no sensor, but only a blank plug, then your tachometers are sensing the alternator speed, so wiring, loose belts etc. could be the cause. Like Brooksie said, these tachometers generally have an adjustment.
In either case, you should use a digital tachometer to determine your actual engine speeds.