MySql Qry performance (Ver 5.5.17)

I have the following qry to get the amont & tax where unit_is_pharmacy = 1

Scenario 1

SELECT SUM(Amount),SUM(tax)

FROM

 (
  SELECT unit_code,unit_is_pharmacy,SUM(IF(bad_payer_type = 'A', 0, bad_gross_amt-bad_package_amt - (IF(bah_tran_type = 40103,-1 * IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0),IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0))))) amount,
      SUM(IF(bad_payer_type='N',IF(bah_tran_type = 40103,-1 * ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3)),0)) tax
  FROM
    bill_accounting_details
      JOIN  bill_accounting_header ON bah_rid = bad_bah_rid
      JOIN bill_header ON (bad_bd_bill_rid = bill_rid)
      LEFT JOIN pharmacy_issue_d ON bad_order_rid = issd_rid AND bad_charge_type IN ('D','M')
      LEFT JOIN pharmacy_issue_h ON (issh_rid = issd_issh_rid)
        LEFT JOIN  u_unit drugprocessingunit ON (drugprocessingunit.unit_rid = issh_location_rid AND  unit_is_pharmacy= 1 )
      WHERE bah_effective_date  BETWEEN  '2016-07-01' AND '2016-07-31'
      AND bah_entity_rid = 4       AND bah_category = 'R'  
      GROUP BY  unit_code,unit_is_pharmacy
  ) a  
  WHERE unit_is_pharmacy = 1

This qry runs pretty fast  < 2 sec  


Scenario 2

But the same qry written as below takes ages

SELECT unit_code,unit_is_pharmacy,SUM(IF(bad_payer_type = 'A', 0, bad_gross_amt-bad_package_amt - (IF(bah_tran_type = 40103,-1 * IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0),IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0))))) amount,
      SUM(IF(bad_payer_type='N',IF(bah_tran_type = 40103,-1 * ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3)),0)) tax
  FROM
    bill_accounting_details
      JOIN  bill_accounting_header ON bah_rid = bad_bah_rid
      JOIN bill_header ON (bad_bd_bill_rid = bill_rid)
      LEFT JOIN pharmacy_issue_d ON bad_order_rid = issd_rid AND bad_charge_type IN ('D','M')
      LEFT JOIN pharmacy_issue_h ON (issh_rid = issd_issh_rid)
        LEFT JOIN  u_unit drugprocessingunit ON (drugprocessingunit.unit_rid = issh_location_rid AND  unit_is_pharmacy= 1 )
      WHERE bah_effective_date  BETWEEN  '2016-07-01' AND '2016-07-31'
      AND bah_entity_rid = 4       AND bah_category = 'R'   AND unit_is_pharmacy= 1
      GROUP BY  unit_code,unit_is_pharmacy

Or

SELECT unit_code,unit_is_pharmacy,SUM(IF(bad_payer_type = 'A', 0, bad_gross_amt-bad_package_amt - (IF(bah_tran_type = 40103,-1 * IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0),IFNULL(ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),0))))) amount,
      SUM(IF(bad_payer_type='N',IF(bah_tran_type = 40103,-1 * ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3),ROUND((issd_gross_amt-(issd_gross_amt*issd_gross_amt)/(issd_gross_amt*issd_sale_tax_perc/100+issd_gross_amt)), 3)),0)) tax
  FROM
    bill_accounting_details
      JOIN  bill_accounting_header ON bah_rid = bad_bah_rid
      JOIN bill_header ON (bad_bd_bill_rid = bill_rid)
      LEFT JOIN pharmacy_issue_d ON bad_order_rid = issd_rid AND bad_charge_type IN ('D','M')
      LEFT JOIN pharmacy_issue_h ON (issh_rid = issd_issh_rid)
        JOIN  u_unit drugprocessingunit ON (drugprocessingunit.unit_rid = issh_location_rid AND  unit_is_pharmacy= 1 )
      WHERE bah_effective_date  BETWEEN  '2016-07-01' AND '2016-07-31'
      AND bah_entity_rid = 4       AND bah_category = 'R'  
      GROUP BY  unit_code,unit_is_pharmacy

I ran the explain Select - and the o/p is enclosed for each scenario.  

Request an expert to  explain the  difference in the qry performance.
Explain-Qry-op-mysql.xlsx