The number the importer displays is not the actual number of records. Its a count of the total number of record id's scanned.
For example if you have 3.5 million posts records, but the highest post id is 5.2 million, the counter will go up to 5.2 million. And it gets even further from the actual number if you're running a multi-threaded import, it stops and you need to restart it --- some of the threads will keep bumping up the count as the skip over records already imported.
Basically, don't worry about it because it's not always an accurate reflection of the numbers of imported records.