I was referring primarily to the weird use of a Roman numeral: "starting XI."
Ah - I see...In the same way that 'soccer' was used to differentiate from'rugger', so the use of numbers became journalistic shorthand (when most sports reporting was written) to allow the reader to easily identify which sport was being reported. So 'starting XI' for soccer, 'starting XV' for rugby union, 'starting XIII' for rugby league, and so on (professional rugby league players still wear a '13' or 'XIII' patch on the sleeve of their jerseys).
Unusually, Aussie Rules commentators generally don't differentiate between those out on the field, and interchange players - they refer to the 'starting 22' (the whole playing squad)...
(edit) at the time the use of numbers was adopted - late Nineteenth Century - the use of Roman Numerals was far more common than today. I mean, who'd call the Superbowl 'Superbowl LVII'?...
