While UCLA and USC are generally seen as roughly similar in admission selectivity, they do look for different things, so it is not a given that you will have the same results at both. One of the more obvious differences is that UCLA emphasizes GPA more than test scores, while USC emphasizes test scores more than GPA (relative to each other), which can be seen from their frosh profiles (UCLA frosh have higher HS GPA, while USC frosh have higher test scores). So if you were GPA-heavy relative to test scores, your result is not surprising. But test-score-heavy applicants may be more likely to get into USC but have disappointing UC results.