Just like those word puzzles, your brain can still read; people fill in communication gaps using past experiences, often leading to misunderstandings. If you're not clear, others will guess... and not always correctly. Say it plainly the first time.
So what happens is people, if you ever see those puzzles where you have words in a paragraph and some of the letters are missing and you can still read it, and it's like a mind strength exercise, well, that's what people do with communication verbally, non-verbally, written or not, is you think to yourself, okay, I'm going to fill in the gaps based on my past experiences and we're gonna take it from there.
And I'm either gonna ask for forgiveness or ask for permission in the future, and I'm not sure which one yet. So that's what most people do unless it's explained to them clearly.