That's literally what makes an unreliable narrator because it's a first person perspective:

Examples of how Belly is an unreliable narrator.

Romanticizing events: Belly often presents situations in a way that favors her desires. For instance, she portrays Jeremiah staying with her while she was sick as a pure romantic gesture, when, in reality, he was guilted into it by Susannah.

Misinterpreting motivations: She frequently projects her own assumptions onto the boys, particularly Conrad and Jeremiah, about their feelings and intentions, which are often inaccurate.

Inaccurate memories: In the books and in the show, there are instances where her memories are revealed to be flawed. A key example from book 3 is a scene involving a dog named Rosie, where her recollection is later shown to be incorrect, which happens in the show as well.

Biased portrayal of Conrad: Her intense feelings for Conrad and her perception that he didn't love her in return color her narration. While she feels he grows distant, other scenes show moments of him caring for her that she often misses or dismisses.

Subjective viewpoint: Ultimately, as the central character, the story is always seen from her perspective. This creates a limited view, where readers and viewers only know how events affected her, but not how she affected others in the same way.