Interpretations differ because they are written for different audiences. Historians select information and when they write they can distort information to make their arguments stronger. Historians change their views when they discover new evidence.
The correct answer should be "Moral obligation to bring Western Civilization to less developed areas".
This was a justification and the way they presented it in public. It was especially prominent in Spain as they said that they were expanding for Christianity whereas they really wanted gold from colonies.
