Well, your question is most valid, I have also notice that a few times that I checked out and while I don't have an answer to your question, I can say that, a great number of those "visitors" are people that while purchasing or enquiring about some eof my products or even asking general questions about pen turning related, sent links to the IAP appropriate threads or post, regardless if they are mine or someone else's.
Many have become members as a result but, they may be reluctant to join in, immediately, they may look around for a little while before they make the decision.
In some Forums, you have to join in to be able to see the threads/post and particularly the pics they contain, I'm not sure that, this is the case on IAP, I believe is not but that, Jeff or one of the Mods, will be able to answer...!
However, I should say that, I may be responsible/accountable for "some" of these daily visitors buy I doubt that, they would be that many in a single day...!
Promoting IAP, that's what its called...! In my view, off-course.
Cheers
George