You say the job is really easy, and your manager rates you a 4/7. Maybe you’re playing the game in easy mode? I.e. doing (well) what’s asked of you but not doing the implicitly expected (finishing tasks earlier than you said you would, writing a post on the company blog, volunteering for pager duty, whatever)
To be clear, it’s absolutely your manager’s job to set expectations and to walk you through the cultural norms of the company, but bad managers happen. Maybe it’s worth having a chat with some of your colleagues who’ve been in the company for a few months to understand how their integration went and see if they have some tips for you (assuming your manager isn’t more forthcoming on how to get a higher score).