Contact normal stiffness affects both accuracy and convergence behavior. A large value of stiffness gives better accuracy and less penetration, but the problem may become more difficult to converge. If the contact stiffness is too large, the model may oscillate, with contacting surfaces bouncing off each other. With large normal stiffness, the normal contact force will be high, and because of which contact was unstable. We generally need to find a balance between allowable penetration and accuracy needed to decide the normal stiffness factor. You can also check below Ansys How to video on interference fit.