What version of NGUI are you using here, and is the UIGrid the only method of positioning you're using or do you have something anchored? UIGrid's Reposition() function should do the job if it's the only thing you have.
3.5.5
The scroll is anchored
The grid inside the scroll is not anchored
The item (sprite) is anchored to the scroll (Advanced left/right binding only)
The pictures inside the item (this is where the problem is) are anchored to the item
I am calling UIGrid.Reposition() after I add the items to the grid.
I am calling it 1 frame after the items have been added.
This is a strange bug... being that it fixes itself If I change to the scene tab or activate/deactivate the object.