It's not really as cut and dried as you make it sound.
Well that depends on how you describe lucky, he plead guilty as part of a plea that offered a lighter sentence, and then ended up getting the exact same sentence as the person who actually did the stabbing, is that really "lucky"? Should he have got a longer sentence than the person who actually killed the guy? It seems that he was lead to believe that if he plead guilty he'd get 7-11 years, so really all the Arnie did was commute the sentence to that which was already promised as part of the original deal.
While involved in the fight, he didn't actually kill anyone.