Django annotation is giving two diffrent result in two cases

Question

I have two cases:

Case 1:

qs = Stockdata.objects.annotate(sales_sum=Coalesce(Sum('salestock__quantity')))
qs2 = Stockdata.objects.annotate(purchase_sum=Coalesce(Sum('purchasestock__quantity_p')))

Case 2:

        qs = Stockdata.objects.annotate(
           sales_sum = Coalesce(Sum('salestock__quantity'),0),
           purchase_sum = Coalesce(Sum('purchasestock__quantity_p'),0))

The result which is coming in Case 1 is absolutely perfect but when I am trying Case 2 the result is coming multiplying the no of entries I made..

Can anyone tell me what is the reason for this and can give a perfect solution to rectify the errors because the 2nd Case is what I need to use in my project...

Thank you

Edit

I have tried this according to the response:

        qs = Stockdata.objects.annotate(
        sales_sum = Subquery(
            Stock_total_sales.objects.filter(
                sales = OuterRef('pk')
                ).values(
                    'sales'
                ).annotate(
                    the_sum = Sum('quantity')
                ).values('the_sum')
            ),
        purchase_sum = Coalesce(Sum('purchasestock__quantity_p'),0)

But having a bit problem of using "the_sum" in further subtraction of annotated values..

I had done this previously:

        qs1 = qs.annotate(
        difference = ExpressionWrapper(F('purchase_sum') - F('sales_sum'), output_field=DecimalField()),

    ) 

Anyone any suggestion what should I use instead of sales_sum.

Thank you