Earlier quoted context omitted.
"1 in Y" is simpler than "X in 100". 100 introduces an irrelevant big number.
I think OP's intuition is probably wrong and that "1 in Y" is more effective than "X%". That being said, it is tricky to move the adjustment between the numerator and denominator, and there are cases where this choice can be key -- "25 miles per gallon" is indeed a far worse metric to use than "4 gallons per 100 miles".
You see (at least in the UK) dishwashers say they only use x litres of water per wash or even things like "this only costs 10p an hour running costs".
It feels like one less step to get to "how much would this car cost me for my 250 miles a week" - which seems more common than "I can afford exactly 2 gallons of petrol a week, how far can I get with this car".
Maybe I'd feel differently if we bought fuel in the same units we measured its consumption in!