The maximum value of Mises stress attained from contour plot isn't equal to that from the .odb field output, but why? 

for instance:

it's easy to find that when the contour plot shows that the max value is about 50MPa, the .odb field output curves demonstrate only about 15MPa at the same time. Field output curves are the max envelope of the integrate points/unique nodals of the whole model.

thank you for your attention🥺