When looking at a leads email activity, you see that they clicked a link in an email, but there is no open activity recorded for that same email.
It is technically possible for a person to click a link in an email without opening it. An Open Email event is only logged when a single pixel tracking image is downloaded from the Marketo server, but many email clients, such as Outlook, don't download images by default. So someone could open the email, read and click it without choosing to download the images, resulting in a Click Email without a corresponding Open.
Email performance reports have an additional logic in them that backfills the Opens. They know that to click an email you must open it. Smart Lists don't have this backfill, so the Opens in a Smart List will often be different than the Opens in a report.