Why is measuring email open rate challenging?

Question $8$ options:

$1)$

Because people can open emails without actually seeing the content.

$2)$

Because some people will open an email but not click through to the content.

$3)$

Because some people have images blocked and thus the invisible image used for tracking is blocked.

$4)$

Because some email clients $($like Gmail$)$ group all emails from a sender together and only show the first one, making it look like the email wasn't opened.