NURCaptcha inserts a reCAPTCHA on the Register Form of your site to protect it against spambots.
If you was facing a greater number of spambot registrations than the figures you see now in the log counter, that means the plugin is doing its job perfectly. Almost all the spambots give up their attack when confronted by the reCaptcha challenge, so they come and go silently. These bots may not trigger the counter, but they will not get inside your site walls, either.
That is the main target of any captcha plugin: to catch bots and allow human beings to pass by. In order for it to somehow prevent you against badly mooded human intruders, their data (email & IP) are checked against trustable anti-spam databases (from plugin version 3.0.0 on).
These people will leave their track on the log file. So check the log file regularly to see if there is someone you'd like to bring into your subscribers database. The log will give you that person's username, email and date/time when he tried to register and failed. Then it's up to you to add that user manually or send him an email with directions for an effective registration. The most common problem that keeps good people outside is not due to the reCaptcha, but illegal (non ASCII) characters in usernames. That is why we added a help box to the register form.
The reCAPTCHA form that NURCaptcha inserts in your site is built up on the fly at Google`s reCAPTCHA server. NURCaptcha works, to this moment, with the standard ready-to-use four basic styles. We plan to add customization tools on a future version of the plugin.