Help With Inner Join

Hi,

Can some one explain me this issue

SELECT COUNT(*) FROM "ECCDATA"."CUST1"  O/P = 800

SELECT COUNT(*) FROM "ECCDATA"."CUST1"  WHERE MANDT = '800' O/P = 800

SELECT COUNT(*) FROM "ECCDATA"."CUST2"  O/P = 13779

SELECT COUNT(*) FROM "ECCDATA"."CUST2"  WHERE MANDT = '800'    O/p = 13751

SELECT COUNT(*) FROM ( SELECT T1."A1", T2."MANDT" FROM "ECCDATA"."CUST1" AS T1

INNER JOIN  "ECCDATA"."A2" AS T2

ON T1."A1" = T2."A1" AND

   T2."MANDT" = '800' )      O/P = 13783

SELECT COUNT(*) FROM ( SELECT T1."A1", T2."MANDT" FROM "ECCDATA"."CUST1" AS T1

INNER JOIN  "ECCDATA"."A2" AS T2

ON T1."A1" = T2."A1" AND

   T1."MANDT" = T2."MANDT")      O/P = 13779

Thanks