TERRAIN FROM ROP MESH - TERRAIN ERRORS

 I created a terrain from a corridor top mesh using the following ORD tool:

   

The mesh sits on top of the corridor precisely. However, the created terrain does not align with the mesh. 

I.e there is a miss-match below where the created terrain does not align with the mesh

Another example is seen where the created terrain does not pick up the left edge of pavement 

Is there a reason why this may be happening and a solution around it?