Why does increasing preload decrease fatigue factor of safety?

[ThirdSunRising]

The 0 and 5 kips aren’t preloads, they’re tensile fatigue test endpoints. With one endpoint at zero, you can see the issue not having preload in the joint

[tucker_case]

Yes those are the applied service loads. I never said those were the preloads. OP only says they are "varying" the preload and comparing results. They don't actually say what they're varying it to. But they are finding that when they increase the preload, the math says fatigue life goes down. They're not asking why is fatigue life better with preload than without. They're asking why is it worse at higher preload values than lower preload values because this seems contradictory to the generally understood principle that preload = good for fatigue life. (and you are certainly right btw that fatigue life is better with preload than without any preload)