The objects are being distributed by their pivot points, not by their bounding boxes. Since pivot points are dimensionless, it is inevitable that objects that are distributed close to the edges will have some of their parts outside of distribution object. There are several ways you could try to limit this behaviour, but none of them can guarantee perfect result. The methods you could try - 1. use Chaos scatter's edge trim feature, 2. limit scattering near the edges with distribution maps, or with spline include/exclude feature, 3. use Corona slicer to cut away protruding objects (i'm not sure if this would work in combination with the scatter though).
