Grover Cleveland held the post of President of the United States of America on two occasions: 1885-1889 and 1893-1897. As he is the only man ever to serve two non-consecutive terms, he is considered to be both the 22nd and 24th President of the United States.
Thus, President George W. Bush is the 43rd President, but only the 42nd person to hold that post.
Thus, President Barack H. Obama is the 44th President, but only the 43rd person to hold that post.
Since the first Prime Minister of Canada to serve non-consecutive was also the first Prime Minister period, and since a number of Prime Ministers served non-consecutive terms, the number of Prime Ministers and the number of people who held that post remain identical.