How Open Tracking Works

Emails opens are tracked by placing an invisible 1x1 pixel image in the bottom of your emails. When a recipient opens your email with images turned on, the image loads and pings the email server. We instantly count and record this as an open. 

Opens are distinguished in Reports between the first "Unique" open and additional opens that count towards your "Total" opens. You will see 'Unique' open counts and total open counts in the embedded Power Bi reports.

Why is there so few Opens?

If a recipient simply reads the message, without allowing the images to be displayed, the message is not tracked as opened and is never reported back to the email server.

When measuring your engagement, always take into account all reported clicks since a recipient can easily read and click without ever viewing images.

If your emails don't have any images, there's no real incentive for a recipient to enable images. Moreover, when there are no images they can see as part of your message but they still get an alert that the email has images to display, you'll raise a red flag of suspicion. Avoid this! Give them a reason to enable images by including your logo, social media buttons, or images relevant to your content. 

Why is there a High Number of Opens? 

You may see multiple opens from a single recipient for a campaign. Here's some of the reasons why: 

Preview pane causes additional opens

Depending on whether or not the recipient's email client has a preview pane (like Outlook), their opens can be counted multiple times. For example, if a recipient is using an email client with this type of preview pane, and clicks on an email once, it will appear in their preview pane, which counts as one open. If they double-click on the email to open it in its own window, that can count as an additional open. Every time the recipient is browsing through their email and clicks on that email, it will count as an open because the invisible image is loaded multiple times.

Viewing on desktop and mobile device

In some cases, recipients will load the email on their desktop AND on their mobile device. Each time a recipient views the email on a new device, it counts as an open.

If a recipient uses their email client to forward an email to other email addresses, the tracking links in your campaign will remain the same (as we have no way to change it). Every time a recipient of the forwarded email opens the email, it will be counted for the original recipient that received it. For example, if you send Bob an email and Bob opens it once then forwards it to 5 friends, and they all open it, it will show up as 6 opens for Bob's email address.

Email security scanning/Spam software

Some email scanning programs or security filters can trigger an open. Basically, if anything attempts to load the tracking image in your email, it will be counted as an open, so it is possible for something like this to trigger an additional open (or many!).

Source - https://jangomail.zendesk.com/hc/en-us/articles/219089147-Open-Tracking