That is how Family Tree Maker creates these Descendant Charts.
BUT, here is the reason:
You observed it, but not sure you caught the drift:
"child it is shown underneath the couple with a single line connected to either the husband or wife ...sometimes to a wife and on other parts of the chart a child line is connected to a husband"
I think that if you look, you will see that the "single line" is drawn down from the non-direct line spouse. This is important when the Direct Line Spouse has multiple marriages. You can visually see which relationship created the children, but related to the Descendants in that Chart.