Discrepancies in the values of standard atmospheric pressure: the riddle resolved

Over half a century ago, when I took Beginning Physics at university, I was bothered by the anomaly that normal air pressure (which is arbitrary anyhow) was taken to be 1.013250 bar instead of a sensible 1.000000 bar. My professor could not explain why, nor could any of the many other scientists whom I have asked since then. Now in retirement, I have made a laborious search in utterly uncharted territory and discovered the answer.