I'm only logged in on one computer. On the others, I just browse the publicly available areas. There is still plenty to read there so it is conceivable that someone might register in anticipation of becoming an active poster but then just lurk for a while and not bother to login until something compels them to jump in and post.
But, I would bet that most forum registrations are bots. That was the case when I had open registrations on my forums years ago and I'm sure it is even worse now.
For people that register and don't activate, on the registration success page, I would put up a notice that they will receive an activation eMail and they'll need to click that link to finalize their registration and that accounts not activated within 10 days will be deleted. Then, information on how to contact you if they don't receive the activation eMail. Of course, that's assuming you don't have something like that already.
I know that sometimes when I've had problems registering on a site, I might wait a day or so before contacting the site just because I get busy or whatever. So, 7 or 10 days or something gives legitimate people trying to register a good amount of time to complete the activation or contact you.
Also, deleting incomplete registrations also prevents problems with people trying to register and then the system saying, well, you're already registered. Or whatever. I've had that too where I couldn't register because I didn't receive the activation eMail and then couldn't try again at a later date because the account I tried to set up was still there.
Also, there's a trend these days where people don't want to have contact with actual people, so having to send you an eMail to activate their account is going to be a no-go for them.