And the origin of the phrase is a disclaimer on gas (petrol) mileage estimates for automobiles, at least in the USA, in decades past. It was a common tag in TV ads for cars after touting an "EPA estimated highway and city mileage" for that car. (EPA being the Environmental Protection Agency, which issued the standards for ratings gas mileage for vehicles. Those standards were based on rather theoretical tests, which many people didn't match in real-world driving.)

In other words, it doesn't refer to airline miles. It's slang for "you may get different results".

(And the FT glossary doesn't eplain any of that.)