The thing about Harry Potter as a character is that he is insanely observant when he actually cares enough to pay attention. Meaning 90% of the stuff he deems unimportant flies over his head, but he makes these huge leaps of logic and intuition when he bothers to focus. Like in the books when it comes to anything relating to Voldemort or Death Eaters or People Not To Be Trusted (Draco, Umbridge). Growing up, he had to be able to see when a situation was going south long before the frying pan or Dudley’s fists came his way. But he also had to be able to ignore and tune out the constant flow of shit and neglect he was treated to.
If you think about it, for all the better aspects of Hogwarts, it still followed this same basic pattern. He had to pay close attention to the things trying to kill him (even classes took a back seat to this), but find a way to ignore and not acknowledge all the rumors and staring and people thinking he’s a prat or the heir of slytherin or a liar. I think this is why the arguments that Harry is a mushroom and notices nothing, and the arguments that he is deductively brilliant can exist side by side. He’s both. It’s also why, in my opinion, he tends to be ridiculously observant of Ginny once he starts to notice her as something important. She barely exists in the early narrative other than Someone to Be Saved. It’s also why Ginny can sometimes feel like she ‘comes from nowhere’ in the narrative. As far as Harry is concerned, she did come from nowhere. The switch in Harry’s brain went from Doesn’t Matter–Ignore to Very Important–Pay Close Attention, and BAM, there she was. Everywhere.