How are opens tracked?
The standard practice of tracking opens across the email marketing industry is to embed a pixel image into every email that is sent. The image pixel is invisible and it will not affect the design of your emails. Whenever that image pixel gets downloaded, it allows the open to get tracked. If the images on the email are not getting displayed, then the image pixel is not downloading either. This means, that the open will not get tracked.
Why wasn't my open tracked?
When a recipient opens on the email, it will only get tracked if all images are downloaded. For instance, if the recipients settings require them to click on "Show blocked content", then the open will only get counted if the recipient clicks on "Show blocked content"
This will not be the case for all the recipients, it just depends on each individual recipients account settings. Due to this, there will be some opens that will not get tracked.
It says there was an open, but I didn't open it
Many spam filters will crawl links before making it to your recipient's mailbox. Because of this, automated processes may download the tracking pixel we leverage causing an open activity to occur.