Marketo is creating duplicate leads despite they're already in Salesforce with the same email address.

I've noticed that Marketo at times creates duplicate leads even though the leads in Salesforce have the same email address. Today, one of the marketing coordinators imported a list of contacts into a list in Marketo that are already in Salesforce (don't ask), but it created new leads for all of them. One thing I did notice is that she didn't always include the company name. But the email addresses are the same because the list was pulled from Salesforce via a smart list. These are Accounts that are closely monitored, so there's no duplicate in the system for any of these.

So why would Marketo still create a duplicate Lead if the email address of the Contact in Salesforce is the same? I've seen this happen with form fills too where the email address is the same.

I read elsewhere that Marketo might do this if there are duplicates in Salesforce, and although that wasn't the case with the list import above, why would that matter? If Marketo sees at least one record with the same email address, I would think it wouldn't create another one, isn't that right?  Thanks