I suspect that the most important difference isn't even the level of experience with the books--it's with the focus of the attention. Most readers who participate in interviews and chats are doing so for the enjoyment of having briefly touched base with the woman who created something that they enjoy. On the other hand, those whom you call 'devoted fans' are not necessarily more devoted; they are merely differently devoted. Rather than focusing their devotion on the woman and the experience of reading, they instead devote themselves to decoding and revealing more of the world in which the books take place. For these people, the geographers and sociologists and anthropologists of the wizarding world, the story of Jo Rowling couldn't be less interesting. They want names, dates, statistics, historical occurrences, societal factors that motivate actions; they want what you want, and what you feel you aren't getting from the chats and interviews.

Of course, I could just be making things up. ^_~