How Email Open Tracking Works
Open rates are calculated by how many people open an email and display the images (which is normally the default). Email clients don't actually send any information when an email is opened, so all the providers (like ConvertKit) embed a tiny 1px transparent image that is unique to every email. Once that image is loaded the image counts as opened.
That means if someone has images blocked then they never count the email as opened. Though clicking a link will also count as that email being opened.
If the same person loads an email a bunch of times it still counts as one open (since the tracking is unique to each email). So tracking is based on emails opened divided by total sent.