I was just looking at the solid.

If you look at the joined edge, you will see what appears to be a broken edge:-



If you zoom into that area, you will actually see 2 edges with an extra little face.




What as happened is that when you made those 2 surfaces into solids, even though the edge was not in line, the fact that the solids where touching allowed the union, but that then left that little protruding edge.


- Steve