questioning accuracy

It depends....are you Republican or Democrat?  Conservative or liberal?   j/k   But that explains why you see differences in wire charts.  Do you want to approach wiring assuming that the quality of the wire/insulation is the best available, or do you want to take the conservative approach and assume that the wiring might be just average or below grade? 

There ya go....a politician's answer!

But in all seriousness....you are figuring amperage based on RMS power.  In reality there are peaks of higher amperage that will occur.  One chart is a realistic display of maximum amperage per gauge....the other is a reasonable recommendation for wire size based on the novice's need for help in selection.  It is a well-received fact that nothing smaller than 8 ga. wire should be used to run from the engine bay to the trunk to power any amplifier nowadays.