Just a theory here, but the N1 change upon selection of eng synch may be related to the fact that the 731-5 (including the -5BR) family of engines have N1 compensators. A brief explanation of the purpose of N1 compensation:
The fan speed which produces a given amount of thrust varies somewhat from engine to engine. N1 compensators are installed to compensate for this difference. Without compensation, one engine may require a higher indicated N1 than the other to achieve equal (assumed) thrust. Compensation value is determined in test cell runs at the factory. Each DEEC is programmed with the compensation value for it's respective engine and controls the engine N1 ( by controlling the N2) so as to achieve the programmed thrust value. With eng synch OFF, as during T/O and landing, the compensated value is displayed on the N1 indicators. The actual RPM will be different than the indicated RPM according to the compensation value for each engine. Engine synch is accomplished by a synch circuit connected through the DEECs (the DEECs receive uncompensated N1 signal) by comparing the raw, uncompensated RPM of each engine (either N1 or N2) in order to synchronize the selected rotating groups by altering one of the engine's N1 or N2 (as selected) and achieve the most comfortable vibration characteristics.
The XP I used to fly did the same thing. My theory is that when the synch is selected on, the N1 guages now reflect the raw, uncompensated value. This is what would interest you for the purpose of RPM synchronization, where the compensated value displayed for takeoff is more useful in achieving an assumed value of thrust to meet certified takeoff performance.
If the compensation value of either engine is signifigantly different than that of the other, the master engine N2, Wf (fuel flow) and ITT will remain stable while the slave engine will shift slightly to match either the N1s or N2s according to the synch switch position. N2, Wf and ITT must change to reflect any ACTUAL change in N1. You are most likely seeing the difference between compensated and actual N1 when you switch the synch on. If no shift in N2, Wf and ITT takes place, it probably means that your engines have nearly equal values of N1 compensators installed. So you see only the difference between compensated and uncompensated N1 value. Less equally compensated engines may, when RPMs are matched, exhibit noticably unequal thrust. The plane I currently fly requires a notable rudder trim offset when N1s are synched. Oh, well...
The 800XP is a pretty good ship. Enjoy her!
Best,
