Why are stats in Angband written as 18/** instead of 19, 20...?

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In Angband (and a lot of derivative, and maybe in its predecessors also), when your stats (e.g. DEX) is below 18, it is written with a single number (1 to 18), and after that, if the stat still increases, it is written with the following format: "18/xx", where xx goes from 10 to (more than) 200.
Also, xx increases by more than 1 when you get a +1 item.
What is the intuition behind this notation?
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Ch 18 22 Summary AP Stat
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