I think Verno's idea has some merit too -- the fact that Optimus Prime and G1 Megs' Sparks were dormant at the time, whereas Starscream's was very much active. Cross that with the mutation whereas G1 Megs' Spark probably isn't, and we know for sure that Optimus Prime's isn't (cos we've seen his Spark return to the Matrix in G1, especially when he died and Rodimus Prime was running around as leader)
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