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I think it's only because of the rythm. Like in the chorus of decision, where she's singing "Sou boku wa yuku...", it would sound funny if she sang "Sou watashi wa yuku...".
The same when singing bokutachi in the last chorus of rainy day and talkin' 2 myself, it would sound funny with watashitachi, right?
She simply use what's best in the music. In some songs, she use both watashi (or atashi) and boku, anata and kimi.
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