There was a time lapse between the Gabriel's announcment to Mary and His assurance to Joseph. Why?
For both of them to be informed at the same time would have saved some tense moments for them both and for her family, I'm sure. But, it didn't happen that way. For some divine purpose, there was a lapse. Heaven knew why. And Heaven knows why we face lack of knowledge in some facet of our understanding about life. God was faithful to inform Joseph at the right moment; we can trust He'll do the same for us.