Hi Rotem,
That does seem like an unusually high number of unconfirmed subscriptions.
Since I don't really know the nature of your demographic, the best I can do is offer some advice that's worked for me.
On my sign up pages, I mention that the list is double opt-in and that they will need to confirm their subscription to start receiving the mailings.
On the thank you page, I include a screenshot of what the confirmation e-mail will look like with an arrow directing them to click on the confirm link.
I also mention that they should white list my e-mail address (i.e. whichever e-mail the confirmation e-mail will come from) and I include information on how to contact me if they don't receive their confirmation e-mail.
If you're including a bonus for subscribers, make sure that the confirmation e-mail doesn't contain the download link (only the e-mail that gets delivered after they confirm).
I've also published some additional tips for
increasing newsletter confirmations.
Hope that helps,
~ Teli