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.