This also has to do with how Marketo (and email clients in general) record an email open. There are 2 ways:
1. If the user downloads an image (or all the images) in your email, Marketo marks an "opened email" for you. These opens will show up with a smartlist filter
2. If they don't download images, but clicked a link in your email. This is an indirect way for Marketo, and you will not get these opens when you use a smartlist filter.
On the other hand, if you go to reporting and create a report where in "Setup" (instead "Smart List") you add a filter for only the one email you're looking for, Marketo will open a report with every open. That is, those who downloaded your images + those who did not download images, but clicked a link.
Finally, if a user did open your email, but did not click a link and didn't download the images, you will never know they opened your email. Unless they reply to you .
Hope it helps.