Why mysql query returns a different result?

combine two tables using the below select query:

SELECT item_name,batch_no,invoice_no,invoice_date,damaged_qty,qty,unit_vat,unit_discount,unit_price,packing,free FROM
(SELECT item_name,batch_no,invoice_no,invoice_date,damaged_qty,qty,unit_vat,unit_discount,unit_price,packing,free
FROM damage_stocks AS S UNION ALL
SELECT item_name,batch_no,invoice_no,invoice_date,qty,qty,unit_vat,free,unit_discount,unit_price,packing
FROM purchase_invoice AS D)  AS alias_table
GROUP BY  invoice_no,item_name,batch_no
HAVING COUNT(*)=1 and invoice_no=1
ORDER BY item_name,batch_no,qty;

Resultant output :

'MM TONE_', '1', '1', '2015-06-24', 10, 10, 0.00, 2.00, 0.00, 20.00, '10'

You see, it displays free as 10 but in my damage_stocks table, free column contains no value and in the purchase_invoice table, the column free contains 2 for the invoice_no 1.

select free from purchase_invoice where invoice_no = 1;

output:

2

this

select free from damage_stocks where invoice_no = 1;

returns an empty value, because there are no values are added.

And my question is,

• Why free column displays 10 instead of 2, since the free column in purchase_invoice contains 2 for the invoice_no 1 ?

• How I get the output as free col's output as 2 instead of 10?