Hmm.. I don't know, Remi... it seems they have their own business logic, with months absolutely independent on real calendar maybe setting things right would create confusion.

If we suppose that you don't want to (or can't) change anything, try this:

SELECT Q.city_code, Q.outlet_code, SUM(Q.sold0104), SUM(Q.sold0508),SUM(Q.sold0913),SUM(Q.sold1417),SUM(Q.sold1821)

FROM

(select [city_code], [outlet_code],

CASE WHEN issue BETWEEN 1 AND 4 THEN sold_units ELSE 0 END as sold0104,

CASE WHEN issue BETWEEN 5 AND 8 THEN sold_units ELSE 0 END as sold0508,

CASE WHEN issue BETWEEN 9 AND 13 THEN sold_units ELSE 0 END as sold0913,

CASE WHEN issue BETWEEN 14 AND 17 THEN sold_units ELSE 0 END as sold1417,

CASE WHEN issue BETWEEN 18 AND 21 THEN sold_units ELSE 0 END as sold1821

from [MerchDB].[dbo].[tblOutletSales]

where active = 'yes' and issue_year = 2004) AS Q

GROUP BY city_code, outlet_code

ORDER BY city_code, outlet_code