Why do some copies of _Pride and Prejudice_ have 61 Chapters and Others Three Volumes?

I looked into it a bit and found that the novel was originally published in 1813 in three separate hardback books.  That accounts for the volumes.  Later, when the book could be easily published in one volume, it was.  Some copies have the original divisions (based on the three books) while others number the chapters with no break. 

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