Hi, although the result seems wrong, really in this case the distribution is correct: when a surface is used as scattering area, inner boundaries are considered as Exclude areas and Forest creates items around them because the "Exclude Fallof->Inverted".
I suggest this alternative method, which has not that problem:
1) Create a spline surround your surface. Its shape is not important, you can use a simple rectangle.
2) Assign it as Include area. Be sure to move it below all Exclude areas in the list (the order of Include/Exclude areas is significative).
3) Turn off the "Surface area", so it doesn't generate items.
With this configuration, Forest uses the surface only as a reference to align the items on Z, but not as a scattering area.