Actually there are a lot of experts who think
Nancy was first a pet form of Annes and only later applied to
Ann. That would explain where the -s- sound comes from in
Nancy. Originally the pet form of
Ann would have been
Nanny and of Agnes/Annes would have been
Nancy, but as many people in England completely confused
Ann and Annes, there being many examples of the same woman referred to as
Ann in one record and Annes in another,
Nancy came to be applied to
Ann as well as Annes.