You know what you mean to say. It makes perfect sense to you in your head, and it has made sense to many others before. It has always produced the desired reaction, a simple phrase that needs no clarification:
"I'll do it soon" or "I'll write a detailed report."
You assume that you are now on the same page, but that is far from always the case. There are so many phrases that we use in our daily vocabulary that mean one thing to us, but another thing entirely to someone else. When perspectives on language differ, the gap in understanding can be a surprisingly wide gulf.
In a world where no one has much time to pay attention to the nuances of language, it is so easy to interpret "soon" according to our own frame of reference rather than thinking of what the other person might mean. Chatting about a project in the corridor and confirming a detail with a quick clarification is one thing, but would you really make the effort to reply to an email with the question "when exactly do you mean by soon?" Probably not.
The onus is on the communicator to be clear with their language in the first place. Where relationships are relatively new, more detail is definitely better than less, and it may be worth slowing down the flow of information that little bit, just to make sure that they are still with you. When you leave someone behind in your conversation wake, as they try to make sense of what you have been saying, they are not processing the extra stuff that follows swiftly after.
You have to understand where to pitch the conversation, understanding their level of familiarity with any given issue before you jump right in to solving it. No message should come in a one-size-fits-all package - you have to ask some questions to understand how it should be wrapped. Wrapping something carefully always takes that much more time and effort, but the end result will be a clarity that can truly be relied on.
The mother of assumption is laziness. It is so easy to use fluffy corporate speak that doesn't really mean anything and can be deciphered in so many ways that you can squirm out of many sticky situations. "I said soon, but I didn't mean this soon." When on the other hand, if you are laser focused with your deliberate language, people will see you as someone who knows their mind and who can be trusted. Now all that remains is delivering on your well-communicated promises.
Language is only effective when you get an appropriate response. For that response to take place, the other person involved has to understand what you mean. Not just the meaning of the words, but also what lies behind the words.
Sometimes you simply have to spell this out.