Topojson.merge produces a difference instead of a union

[DenMesh]

geometries.txt
The attached file contains two geometries that behave weirdly. As far as I was able to confirm, both are similarily winded. However, the following code produces, weirdly enough, a DIFFERENCE, not a union:
var topo = topojson.topology({ data: FeatureCollectionOfGeometries }); var geo = topojson.merge(topo, topo.objects.data.geometries);
Result was checked by projecting the data like this:
var projection = d3.geoAlbers() .rotate([-105, 0]) .center([-10, 65]) .parallels([52, 64]);
(and a fitSize)
and then drawing it as an svg path with d3.