The video evidence showed that Krasic dived. The referee didn't see the dive and gave a penalty (he didn't punish Krasic). The ref two days ago was told that Cavani elbowed Chiellini and he decided that Cavani should be yellow carded, or in other words, the referee already punished Cavani so now video evidence can't be used, which wasn't the case with Krasic and his dive.