@Tersione It likely just broke at some point during her wanderings, she does state she's an old unit, and she replaced it with a different product than what was already there, like when you replace a lightbulb in a fixture but don't match the brand so you end up with one bulb that looks different from the others in the array.
This is deeply sad, but also deeply moving. The little girl gave this notebook to the robot so that she could go and do a bunch of fun things. This wasn't a final task, but a final gift.
@nyanpass Wear and tear, a mismatched replacement part 'cause they don't actually make this model anymore, special feature in one eye only, or just straight up the designer's/previous owners preference. Take your pick.