I thought it was patent that 3 x 22 x 10 x 8 = 5280. I included the derivation for amusement.
Furlongs.
Historically standards like this have been adapted to make the measure fit something that already existed, in this case the Roman mile.
A favourite example is the tonne which is the mass of a full 900 litre wine barrel (pl tonneaux) which is why it's almost the same in English and metric units. In English they tweaked it up to fit an even number of stones, in metric they tweaked it down to fit an even number of litres of water.
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