Possible causes
If the number of opens differs from the expected number
1. Sending text messages
HTML tags (1-pixel images) are automatically inserted to measure the opening rate. This means you need to use HTML mails for this function to work properly.
2. The recipient's email or security software doesn't allow HTML
As with the above #1, HTML must be allowed for the opening rate to be measured.
3. Security software is getting in the way
Some security software will scan images. This will essentially open the above-mentioned HTML tag, which can give inaccurate opening rates.
4. The mail was opened very recently and the open hasn't been recorded yet.
If the number of clickthroughs differs from the expected number
1. Part of the URL is disabled in the recipient's email or security software
This disables the tracking function, so clickthroughs won't be tracked.
2. Security software is getting in the way
Some security software will click links. This will give inaccurate clickthrough rates.
3. The mail was opened very recently and the clickthrough hasn't been recorded yet.