[jcranmer]

My gut reaction is that the probable cause of the discrepancy is ultimately "someone calculated something wrong." It's not like particle physics experiments actually generate something directly measurable like the muon's magnetic moment; you instead get an indirect result that you have to calculate to the value you're trying to measure. And half of the constants in that calculation are themselves from reported results in other experiments, which similarly returned indirect results that are dependent on correct results from other experiments... and it's not hard to imagine that the result of "value X is A" depends on an assumption that "value X is B" somewhere in the calculation tree.