Hi,
There are two ways to see if an email has been opened. The first one is the use of a trackig pixel which will be loaded from your webserver end the second way is click tracking.
While click tracking is fairly reliable, the tracking pixel is not. There are two reasons. The first reason is when a receiver reads your email without displaying images (keep in mind that moset email clients do not display images by default) this is not counted as an open since the tracking pixel was not loaded.
The second reason is that some email providers cache images and they even recognise tracking pixels whch then are also cached and reused in case sonone needs to display the tracking piixel. Apple secure email is using this for example.
In case the (tracking) images are cached you only see the initial loading in your statistics. Hence, you see only one open.
The tracking pixel and thus the open rate is becoming less and less reliable. Though the open rate is still a good indicator to compare different emails with each other. For example, It can help you to identify if certain chanhges in your email or using different sending times result in a diffeent open rate.
