Different designs. ARC doesn't have individual fuses on each tube, like some tube amps have, as they think it affects sonics. Also, it could be partially circuit topology, which can affect how the unit reacts to a voltage or current surge.
tube/resistor failure
I was just reading the Stereophile test of the Audio Research VS110 and they had an output tube arc/fail which also resulted in a burned out resistor. I have had output tubes fail in my amps (VAC and Cary) without any resistor damage. Have I just been lucky or is there something unique to the Audio Research designs that automatically results in resistor damage if a tube fails?