This trigger is supposed to ajust the quantity in the IN_ItemEntrepot table.
The logic behind it is: when updating the received quantity in the IN_ReceptionDetail table the trigger should substract the actual quantity and add the new inserted quantity.

So if in hand is 100


updating the received quantity from 50 to 75 should do the following:

100 -50 + 75 = 125

What I think is really happening is 100 -75 +75 = 100

What is wrong here?

CREATE TRIGGER [ReceptionDetail$UPDATE] ON [dbo].[IN_ReceptionDetail] 
FOR UPDATE
AS
IF UPDATE(nQuantiteRecu)
  BEGIN
    UPDATE  IN_ItemEntrepot SET  nEnMainQuantite =  nEnMainQuantite -   
    IN_ReceptionDetail.nQuantiteRecu + INSERTED.nQuantiteRecu FROM  
    IN_ItemEntrepot 
    JOIN INSERTED ON  INSERTED.fk_Item =  IN_ItemEntrepot.fk_Item AND  
    INSERTED.fk_Entrepot =  IN_ItemEntrepot.fk_Entrepot
    JOIN IN_ReceptionDetail ON IN_ReceptionDetail.ID =  INSERTED.ID
  END