Surface Plot vs. Volume Plot

Hello,

For my simulation I would like to plot the Von Mises stress of a domain in a 3D Plot Group. However, I get different results if I use a Surface or a Volume within the 3D Plot Group. The stress values are substantially lower when I use a Volume plot. Does anyone know why this is the case? And which one should I use for my Von Mises stress analysis?

Thank you very much!