Kevin, just curious if your system is readily able to differentiate between truly unique visitors and search bots/web crawlers. You know much more about computers and programming than I do, but it is my understanding that websites in general are getting more "hits" as these bots/crawlers comb through the site's information more frequently and thoroughly.
I'm a member on another forum too, and they had to block all of these "web bots" because so many were hitting their site. They did not have the servers or infrastructure to handle it. You have a lot of content (forums, articles, and photos), and advertising/external linking, plus your site shows up readily in search engines -- I can only imagine search bots have been all through your servers. In the past, I know it was much easier to decipher the different types of activity, but I've also been told that it's getting increasingly difficult to do that now.
One other consideration is tracking these unique visitors. That's usually done by cookie tracking if memory serves me correctly. As awareness has increased about identity safety, viruses/trojans/etc. most of us have cookies blocked and/or turned off. If I have cookies disabled or I'm using a good firewall/anti-virus software, won't I be considered a unique visitor with each and every click on your site? Also, I visit your site on my PC, on my smartphone, and on my tablet. Even if cookie tracking is still effective, won't I at the very least be considered a unique visitor three times (one each for my PC, tablet, and smartphone)?
I'm not trying to say you're wrong or fibbing about the facts, but like with everything, we all have a tendency to see what we want to see. Plus we all know the cliches about statistics and making them say what you want them to say. I'm just trying to look at this objectively and critically, and I'm trying to do that will all the respect in the world for you, this website, and what both have done for our "hobby". Cheers!