Dear all, thanks for your feedbacks.
After investigating many hours in researching and comparing data. I find out that my customer have many old orders before the migration that are set to "in progress" and not as "delivered". Thats why the big difference.
The original issue is that we have for example in Mai 2016 (when we make the migration), 4 effective sales and when we go in the Prestashop Statistik we have only 3 sales listed. Same for June 2016, we had 41 sales but Prestashop statistic show 40.
Any idea why this can happen ?