stress calculation

Dear all,

Why we have an oscillating behaviour when calculating stress in FEM?

I saw the sentences in the paper:

'Another reason is that stresses are less accurately predicted than strains in FE simulations. Calculated stresses do also show a tendency for an oscillating behaviour, especially in dynamic explicit simulations.'

'stress is less accurate predicated than strain' is easier to understand, but I was wondering the existence of oscillating behaviour for calculating stress.

Any comments are welcome.