Where did that statistic come from?
Not leaving your home town does not make accidental incest more likely. It actually makes it less likely, because you know your own town, (if its small).
Most people know who their relatives are, but not these "mis-fatherd" children.
That is my point exactly. These "mis-fatherd" children will not know who their sibs/fathers are and therefore the likelihood that they will have children together is increased.
The US census bureau posted the 22% figure as those that ever permanently move more than 25 miles from their birth place.