[link to original on tumblr]
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It alternates between two pairs of notes. For instance, after "Girl," it's A & C, Bb & D, and then back to A & C. After "I don't wanna fight," it moves down for F & A, G & Bb, and then back to F & A. After "I'm a little bit wrong," it's back to A & C and Bb & D, and after "You're a little bit right," it's back to F & A and G & Bb. It continues that way throughout the rest of the chorus.
Those two pairs of notes have a note in common: A & C and F & A have A in common, and Bb & D and G & Bb have Bb in common. So, musically, there's an element that's "A little bit me" and "A little bit you too."