You should move the question to Products "Community" is for issues with the Marketo Nation itself -- and yes, this is confusing.
Sounds like you're dealing with a fairly common situation, which is actually by design. By default, as each new person (identified by email address) fills out a form in a given browser session, the web activities in the session -- pageviews and clicks -- get reassociated (linked to) each new lead in turn.
To change this behavior, people use the Referral Form pattern (you can search the Community for examples) so that the web session continues to be associated with one original person.
But even without a Referral Form to use as a kiosk of sorts, Fills Out Form activities should still be in the context of each new/updated lead. What do you mean by "overrides" exactly?
(And also, this has nothing to do with IPs but everything to do with cookies.)