It's perfectly reasonable in general to say it took you 3 years when you weren't actually working on it all that time. Like, say, if you were talking to a friend who you tell everything to.

But when it's in a title or headline, it's implicitly presumed to be the most significant thing about your story to a stranger, thus what makes it notable and interesting. Given that baseline, one tends to interpret it as meaning you were working most of the time on it, because that makes it worthy of attention.

That's why it seems not unexpected to me for a person to feel disappointed and mislead.