Pls move this to the Products space, which is the community tech support space, to get answers.
Whether known or not, once the lead fills out and submits the form they become known to Marketo.
Can you provide further detail and/or screenshots (edited if necessary) so that we can see the Activity Log of the suspect leads?
Could you also provide a link to the form and/or your own submission for us to view?
Consider that leads may use tracking protection features or plugins, which block Munchkin traffic by design. Note Marketo forms endpoints are sometimes (erroneously) blocked by those same features, but in proper operation they'll target analytics hits but not form fillouts.
If you're positive that Munchkin is logging pageviews and clicks for the vast majority of leads, both anonymous and known, then this factor makes up the difference.