Per recent clarification, the Cosmic (Irresistable Attack) enhancement is reduced by the Hardened enhancement exactly as if it were a higher level of Armor Divisor. In addition to that, the Cosmic enhancement on DR allows it to completely stop Cosmic attacks -- in a sense, it acts as "Hardened 6, Only Versus Cosmic Attacks".

So, the only "missing" level of Armor Divisor (looking at the Hardened enhancement) is Armor Divisor (100). It seemed fairest to make it +300%, as dividing DR by 100 is almost as good as ignoring it. It's not quite as effective as Cosmic, but it also isn't affected by Cosmic DR, which seems like an even trade.