It is possible that any of the following factors could have contributed to the inaccurate results when simulated Phishing Attachment campaigns were conducted.
If the attachment is not opened in its native program in order for it to be tracked. (Adobe for PDF attachments, Word for .DOC attachments, and so on)
If the user is using only a preview method to look at the attachment, it'll not count as an opened attachment.
If a user clicks on the link in the attachment preview, this will be tracked as an attachment opened and clicked.