Yes, read through all the resources I referenced in my comments.
In Marketo, the activity log will show you the events that we have recorded. In other words, the technical events that have occurred. However, these events are not good enough to help you understand a mailing, which is why we have additional business logic present in our Email Performance Report. To handle cases, where things don't make sense.
For example, you send an email to a lead and they have "always download images" disabled in their email client. No images are downloaded, causing us to not be able to track an "open" activity, however they click a tracked link in the email. In the activity log, you'd only see the "click link" activity since the open never occurred, but in the Email Performance Report, we would also increment the "open" count because of our added logic. We don't retroactively add activities to the activity log, think of that as more of technical truth for what has occurred for this lead.