Opens are tracked by a special image that is loaded when the recipient opens the email AND shows the images in the email. For that special image to be called however, the recipient has to 'show' the images for your email. In most email browsers, including Microsoft Outlook, image loading is turned off by default. Same or Gmail and many other email systems. Your user can easily click to 'show images' but they may also elect to read your email without showing images, thus making your read-rate lower than it actually is.
The same is true in reverse. Your user could turn on show images by default, meaning that all emails to this group would appear to be read even if they didn't actually open them. While this is far less common, it nonetheless, represents inaccuracies in reporting, specifically at the individual level.
That is not to say that read-rates are not useful. They absolutely are. The limitation of them is more in being able to rely on them at the individual level and that they are frequently lower than actual.