Hey Luke,
When you import a list, you have the option to associate the leads to a particular program. When you do this, the attribution will be given to that program which is a key distinction in reporting. If the record already existed, the record will be given the first status in that program type's progression statuses. If you wish to add records to a list and not have any program status updated, then you would select none for the acquisition program. I have seen this done when people want to import a list, but not add a record to the program unless they were actually delivered an email, for example.
Hope that helps!