Countrows function isn't working

Hello,

I'm trying to count the rows of a certain table as a "measure", like: 

Totalcustomers = COUNTROWS('Sales')

In the query view, I can see that 'Sales' table has 22,083 rows; nevertheless the result of this measure equals to: 13,915.

So, I don't know what is the problem. 

Thank you in advance.