I believe it has something to do with Euro Control having changed the accuracy limits for RVSM and the E145 falls just outside it. Anyone know what the limits are.
Since Mode C gets its info from the acfts Altimeter then it should compare all the time. Unless the test box you mention can measure pressure out side the acft I can't see how it can test the accuracy of the Altimetry. All the altimeters would have been bench tested and can also be tested on the acft.