It’s not intuitively obvious he should be both. GWB is considered to be the 43d President, but he’s only the 42d individual to fill the office. In parliamentary democracies, prime ministers often hold non-consecutive terms and as far as I know the count is generally according to the number of individuals. Pierre Trudeau is considered to be the 15th PM of Canada because he was the 15th individual to hold the office. With American style counting, he would have been both 19th and 21st, if I counted right.