SQL Compare data from two joined tables
WITH
cte1 AS ( SELECT b.team, SUM(a.total) total
          FROM data1 a 
          INNER JOIN team b ON a.teamID = b.teamID
          WHERE a.month = 1 and a.year = 2019
          GROUP by b.team ),
cte2 AS ( SELECT b.team, SUM(c.total) total
          FROM data2 c
          INNER JOIN team b ON c.teamID = b.teamID
          WHERE c.Date BETWEEN '2019-01-01' AND '2019-01-31'
          GROUP by b.team ),
SELECT COALESCE(cte1.team, cte2.team) team, 
       COALESCE(cte1.total, 0) - COALESCE(cte2.total, 0) total
FROM cte1
FULL OUTER JOIN cte2 ON cte1.team = cte2.team
WHERE COALESCE(cte1.total, 0) != COALESCE(cte2.total, 0)


Of course it can be simplified - but it will decrease its clearness.