Short answer:
During a mag check, operating on one mag, the combustion takes longer to complete since there is only one flame front. This results in less total expansion of the combustion gas (PV=nRT) since the thetaPP is occurring later in the power stroke. As the expanded volume of the 3800 degree gas changes, so does the temperature. The result is a higher EGT due to the lesser expansion of the hot gas.
It is NOT because the exhaust valve is opening during the event. By definition, half of the F:A charge is burned at the thetaPP. This is generally occurring at about 15-16d ATDC. In a single mag condition the thetaPP is retarded by about 8 degrees. This means that the combustion charge is completed by around 46-48 degrees after top dead center. The exhaust valve doesn't open until about 135 degrees after top dead center. In a useful LOP mixture the burn completion is later than the example above--maybe a thetaPP of 25 or so, but is still completed significantly before the exhaust valve is opening (about 60 degrees before it opens--give or take a tad). Another red herring of concern.
A more fully descriptive answer as presented in the APS class is not possible in this limited forum post.