nonlinear simulation plot step

Hi All,

I am conducting a nonlinear simulation but found that sometimes the stress result showed is much lower when I modify the model slightly, then I found that the plot step is changed automatically, like the following pictures:

sometimes the time is 1sec, sometimes the time is less than 1 sec and warn "step number shown is of nearest the time available". Does anyone know the reason and difference?

  

My nonlinear simulation setting:

material: 1023 carbon steel sheet(ss)

model type: plasticity - von Mises

Stepping options:

SolidworksSimulation