1st thing to check: "email opens" event only fire if you download all the images in the email. So check that you have indeed downloaded these images. You can open the email without the images and Marketo (like any other emailing solution) will not register the open event, but will indeed register the click, since the click is routed through the click detection server.
Also, please note that you can also register an open if the images were downloaded (for instance by the antivirus, or while the addressee was browsing through his emails) and yet the person has not really opened the email.
This lack of reliability of opens is one of the reasons why it is not recommended to base filter or triggered campaigns on it.