Remember that power is a 'power law' thing. Required power dissipation quickly builds as cathode voltage increases. If the tubes are pulling more that 25v worth of current across the cathode resistor, then dissipation is above 2.5W and there isn't a 2x safety factor any more. Plus a 10W resistor has a lot more surface area than a 5W, so doesn't have the same surface temp for the same heat dissipation.
edit: I don't think this requires a 10W resistor, but I see why these are justifiable arguments.