They don't have images enabled, that's how!
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The open is base on image download. Some email clients do not download images by default and therefore do not trigger an open event in Marketo. But with an email without images, the links are still valid and clickable. So you can click an email without having apparently opened it.
Marketo corrects this in email performance reports, assuming that all clicks are also opens. But this statistical correction is not reflected in the filters and smart lists.