Why did the Early Kings of Gondor live longer than the Kings of Arnor?

[Valandil]

If you have The Peoples of Middle-earth, you'll see in chapter 7, 'The Heirs of Elendil' more information on all the kings of Gondor and Arnor (and its successors) than we are given in Appendix A of LotR. One of the main additions is their birth years. For some reason, Meneldil, son of Anarion, lives to be 280, while Valandil, son of Isildur, lives to be 260. Ages of subsequent kings drop off very consistently, but the northern line drops off even sharper, such that southern kings eventually live 30, 35 or even more years longer than the contemporary northern king (ie, Mallor, b. 895, lived to age 215 - Hyarmendacil, b. 899, lived to age 250).

I find this curious for a few reasons. One, from the lengths of their reigns, which we can easily extrapolate from Appendix A, we see that the northern kings were ruling longer. This led me to conclude that their lives were also longer - but the actual case (from PoMe) is that they consistently waited longer before producing an heir. Another is that in Tolkien, I associate long life with 'blessing' - or even with the 'righteousness' of a king or kingdom, for lack of a better word. I tend to view Arnor and its rulers as more 'righteous' or more deserving of 'blessing' than Gondor and its rulers. But it appears that I'm wrong, either in that association with long life, or in the assumption that Arnor was more so than Gondor. I think I formed the prejudice toward Arnor because; they were descended from Isildur, the elder brother (and therefore were the line of 'High Kings' of all the Dunedain) - and because the line produced Aragorn - and, I don't know, maybe 20th/21st century or personal judgements on Gondor's use of power, etc. Anyway - that's what I thought.

Why do you think this is? Was it just genetic... more 'long-lived' Numenorean genes in Anarion's wife, for instance? Was it the milder climate of the south? Better diet? Did the exercise of power and gaining of strength provide a 'blessing' in itself? Did the shorter reigns lend themselvs to less stress? I wonder if it was the presence of the White Tree(s)... and whatever 'blessing' they may have conferred. Other thoughts? Opinions on options suggested?