Northern Line vs. Southern Line

[Raynor]

Or does the reason lie in the different behavior of both lines?

I agree with that; Gondor resorted to gaining territories, probably levying tributes (seeing how precious stones were said to be pebbles for children to play with) and the sons of Harad kings lived as hostages at the gondorian court. It is worth noting that, previously, the further the numenoreans drifted from morality, the quicker their lives shortened (cf Akallabeth: "and the lives of the Kings of the House of Elros waned because of their rebellion"). Similary, the initial shortening of the lives of Men occured also because they strayed from morality:

Originally Posted by Atrabeth Finrod ah Andreth

Ye have abjured Me, but ye remain Mine. I gave you life. Now it shall be shortened, and each of you in a little while shall come to Me, to learn who is your Lord: the one ye worship, or I who made him.