New Line quantities picked up in Invoicing

Hi

We made a sales order with 3 line items of quantity 8000, 4000 and 2000. We processed the sales delivery and Invoice is generated.

However the Invoice has line quantities as 8000, 6000 and 6000.

User says nothing was manually done to make the changes but wanted the correction to be made.

Please provide the below clarifications:

1. On what circumstances the Line quantities can change in the Invoice

2. What is the fix for the above

regards,