There is another thing to take into consideration, time.
From Appendix A:
Quote:
After the return of Eldacar the blood of the kingly house and other houses of the Dunedain became more mingled with that of lesser Men...
This mingling did not at first hasten the waning of the Dunedain, as had been feared; but the waning still proceeded, little by little, as it had before. For no doubt t was due above all to Middle-earth itself, and to the slow withdrawing of the gifts of the Numenoreans after the downfall of the Land of the Star...
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The more time that passed, the shorter the life span.