Structural calculation difference than principle

Dear all.

In order to know the result makes sense. I made a little experiment on a concrete wall.

The output of maximum stress is way more different than my calculation. 

The dimension is 1m by 5cm, height 1m.

Maximum stress on Karambe shows: 2.24e-3

My calculation is: 1.25e-3

My calculation is as below:

Area: 1000 * 5 = 5000mm

Height = 1000mm

Volume = 5000* 1000 = 5000000mm3 = 0.005m3

Self-Weight = 1250kg/m3 = 1250 * 0.005 = 6.25kg = 62.5N

Maximum Stress(Botton) = F/A = 62.5/5000 = 0.0125N/mm2 = 1.25e-3

Can anyone tell me maybe one of my assemblies goes wrong, or the calculation principle behind the scene causes difference in Karamba?

Best regards,

Shaun