Usually because a Mirror or Array modifier copies the geometry and hands every copy the same UVs. The mesh in Blender is clean; what you exported has two surfaces sitting on the same lightmap texels. LightmapForge measures both the mesh and what the modifier stack turns it into and reports the worse of the two — on a bevelled crate with a Mirror that was nothing overlapping on the mesh itself and 11269 texels shared after the modifier. Apply the modifier before building the lightmap, or leave it out of the export.