You may wish to read ATENA Troubleshooting, 2.1.10 The tensile (or compressive) stresses exceed the tensile (or compressive) strength or yield stress – what is wrong?

I modeled a plane stress model of a simple beam with elastic material using ATENA 2D and compared the results (stresses) with hand calculations and also with the program SAP (structure analysis program ) which used finite element analysis and the following results were obtained:

the stress of the mid span section at top and bottom of the section:

1-for ATENA : stress at top = -6.88 N/mm2 , stress at bottom=6.90 N/mm2
2-for SAP : stress at top = -7.051 N/mm2 , stress at bottom=7.051 N/mm2
3-for hand calculations  : stress at top = -7.025 N/mm2 , stress at bottom=7.025 N/mm2

For ATENA:

Why the ATENA results different from the sap and the hand calculations results?
Why the top and bottom stresses of the ATENA results are different?