My guess is that it probably has a local or dialectical explanation. Here in the U.S., 'bitch' is rarely used to describe the female dog, probably because of it's inclusion in a common curse.. "son of a ------"... It is entirely correct of course, to decribe the female dog with the term, but female dog is more likely to be used outside of dog breeding circles. As a child in the southern U.S., a more polite (somewhat) curse related to that mentioned above, is "bitches welp"...Ironically to your question, this was in an area of the south populated by Scots-Irish descendants. It meant, of course, about the same as the previous, more widely used curse...