This may be due to differences in triggers and reporting and when you ran both reports.
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.