How to use :sum with negatives?

Hi,

I have a database with products - sku and qtt fields.
This is for stock management so when a product goes out I fill the database with -1 (for example)

Then I’m using a RG to show product list with the sum of this values to get the total in stock, as you can see below.

Database:
Screenshot 2022-07-12 at 14.36.44

Values in the RG
Screenshot 2022-07-12 at 14.37.01

Search:

I’m using group by to get all the records by sku and then sum the qtt field results.
The result should be 8 but the RG shows 9.

Can you help me? I’m not figuring out what is happening.

Thanks!!