Why do i get Different Fatigue results by different mesh size?

I'm working on automotive wheel fatigue. I meshed the geometry with Tetra-Volume Tetra (R-trias,Iso layers,elemet size=5)& the fatigue life results is>>>> (min life=0.2, Min simple Average life= 5.9 e 4)
& with mesh element size=6 >>>>>>>>(min life=1.53 e 4 , Min simple Average life= 7.5 e 6)

Would you tell me please Why are the results So different?




I have another question I used the *Solid element* for Fatigue analysis...isn't it better that I use A skin on Solid mesh & i use *Shell elements* in fatigue analysis for better results like it was suggested in Hypermesh tutorial (RD-2070: Fatigue (Stress - Life) Method)?