Dennis, the software doesn't "look" at any screen - only the fields in the table(s) that its algorithms require. I think that what's happening is that the logic that results in decision of half or full relationship between siblings is only looking at whether either father or mother is different for two children. When father is the same, it's not looking for whether father is nonbiological - it just assumes relationship is full (unless mothers are different, of course). I know that if 2 children have same mother, but one has a father and the other has no father, they are shown as half siblings.
On second thought, please scratch my first speculation - the above is a much more likely explanation.