What am I doing wrong in this Update statement

Hi All,

Wonder what I'm doing wrong in this update statement.
The subquery returns the correct no of rows (4085) whereas the whole query when updated returns 3-3.5 times more rows.

update proddta.f1212 set main = 'FDX'
where exists
(select ammin from proddta.f1212
join proddta.f7055 on amdb=cndb
join proddta.f9011 a on a.r6tmt=cntmt
join proddta.f9011 b on b.r6tmt=amtmt
where amtmt < 4500 and a.r6cnt = 'US' and ammin <> 'DHL' and amdb in ('1111', '2222'));

(the am% columns comes from the table F1212)

Thanks in advance for your help to identify.