I think it's to do with the speed of interpretation and thought process. When someone says an idiom, such as 'like water off a duck's back', the thought process goes:
1. it makes no sense in current context, therefore they are not talking literally
2. they are trying to refer to something difficult to explain normally, or using a colloquialism
3. ducks can swim, I know their feathers are treated to prevent becoming completely waterlogged
4. therefore, the feathers of a duck are water-resistant, including the feathers on its back
5. the water therefore easily slips off the feathers on a duck's back
6. the ease of the water slipping off a duck's back is the likely intended interpretation, and because the topic is not about water or ducks, it muse be about 'the ease'
7. They are suggesting that what they are talking about is easy or natural.
It takes a while and repeated usage for me to learn and remember just that 'water off a duck's back' = 'easy or natural'. Perhaps it is faster for neurotypical people to understand them in context, with cues that those with autism might miss.
Ones with semi-logical roots are easier to remember and use than illogical ones. 'Easy as pie', what? As a result I'm usually slower than most people on picking these things up and I get distracted by ones I don't know. I also have issues using them, as I tend to mix them up together.
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ISTJ / ASQ = 37/50
AQ = 143/200 NT = 62/200
?Perhaps one did not want to be loved so much as to be understood.? George Orwell, 1984