Why is there a difference between the turnover overview and the VAT overview?

The turnover overview shows the totals of all transactions based on the session end date. 

The VAT overview shows the totals of what has been invoiced to end users based on invoice date. 

The invoices are sent at the beginning of the following month, which means that the VAT overview can differ from the turnover overview. 

Example: 

Mr. Jansen had a session in March. The invoice for this will be sent in April. 

The session is shown in the turnover from March. 

The invoice is shown in the VAT overview from April.