Write SQL query to solve given problem: What was the average temperature differences during May 2012 for store number 6 and 7?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr AND T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 6 ) - ( SELECT CAST(SUM(tavg) AS REAL) / COUNT(`date`) FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr WHERE T1.`date` LIKE '%2012-05%' AND T2.store_nbr = 7 )