Different nb of visits from campaigns between my dashboard and data exported


I have the following issue regarding the Acquisition module in Matomo, focused on the campaigns part.

When I download all my campaigns from April to June 2019 - I download them week by week as it is not possibile to display all campaigns at once -, I do not obtain the same number of visits from the one indicated in my Acquisition overview dashboard.

In my dashboard I have: 26,332 from campaigns, 36% of visits
In my Excel sheet after having concatenate data : 11 751 visits

Would you have an idea why?

Thank you.