Different people say different stuff because it's a form of Christian mythology, and different people have different explanations depending on where they were raised, the denomination or congregation they attend. It's the same reason why folk songs have many different lyrics, e.g. whether Barbary Allen's boyfriend is named William Green or Jimmy Green or Johnny Green, and whether the rose grows from his grave or her grave. Another popular example would be how the four Gospels do not tell precisely the same Passion story or Nativity -- in no gospel do the "three wise men" and the shepherds appear at the same time.

Last year, I addressed this question as How did the mythology of the Bible develop...?, also featuring the work of /u/sunagainstgold, /u/lcnielsen and /u/idjet. You might like this thread about whether to take Dante seriously.

I have not encountered more answers addressing Paradise Lost since then, so an answer derived from Milton in particular would be good.

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EDIT: This is not /r/eli5, others may go into greater depth than I, aiming for nuance and rich detail.

EDIT2: Found another answer about Paradise Lost/Lucifer by sunagainstgold