A section with unsymmetrical reinforcement will have a pure tension point
with a moment != 0kNm. 
Suppose the top reinforcement in a standard rectangular beam has a lower area
than the bottom reinforcement. When all rebars yield in the pure tension case,
the force in the bottom reinforcement will be larger, causing a net moment.

In the corresponding pure compression case, the concrete will generate a net
moment that counteract the rebar moment. This is caused by the plastic
centroid being located away from the centroid of the concrete alone.
The plastic centroid is the point at which a pure compressive force has its
line of action.