Hi Stijn,
It's possible that the recipients clicked on links in other emails in that same time frame, which the Nurture saw and then sent them the emails. Did you send out another mailing to a similar marketing list at about the same time?
You may already be doing this but I would look at unique clicks by doing an advanced find on the sent emails with that subject and filtering based on Clicks greater than or equal to 1.
Please let me know if you have questions!