It's normal to have more unanswered questions during the private beta. The private beta is a time to establish the scope, level and norms of the site. It's good to have a panel of difficult questions and explore the boundaries of the topic. It's ok if some of these difficult questions remain unanswered until they can have more exposure. It would be worrying if a lot of questions remained unanswered, but 15% is not a problem at this stage, especially on a site like this one where answers are supposed to reflect expertise about very specific topics.
What you experienced on Earth Sciences was the same phenomenon. An expert in a subdomain joined early and asked a bunch of questions. You were quite possibly the second expert on the topic to see these questions, and contributed your knowledge where the first expert had requested it.
Once the site has been public for a few months, 15% unanswered questions would indeed be not so great. The Stack Exchange median is around 7%. More technical topics do tend to generate more unanswered questions than soft topics, because there are more questions that just aren't answerable without a very specific piece of knowledge that can't be faked. The proportion of unanswered questions on Physics.SE is 15%, Math.SE and Math Overflow 20%, Theoretical Computer Science 78%, Cross Validated 35%. History.SE has 6% unanswered questions.