Which created date you are looking at the "Lead Was Created" date, the "SFDC Created Date" or the "Created Date"?
For one of the leads in question, open the individual lead record and check the activity log--see if the lead was actually imported when you think it was.
If it was created via a list import, you should have two activities for that date--the earliest should be
--New Lead, a source of:List Import
Followed shortly by a second
--Add to List, the list name is what you named your import list.
The timstamp should match the "CreatedAt" field.
Basic stuff - but it still isn't accurate - I imported on exactly 12.7 yet the imported date in the activity logs displays 12.10 - unless Marketo took 3 days to import only 55k leads - which wouldn't surprise me. Something else is wrong...