"Total test instances for release" query

Hello,

Anyone know how to get the "Total test instances for release"  ?

My query gives a wrong result (487) :

select count(test.ts_name), releases.rel_name
from test, testcycl, release_cycles, releases
where test.ts_test_id  = testcycl.tc_test_id 
and testcycl.tc_assign_rcyc  = release_cycles.rcyc_id 
and release_cycles.rcyc_parent_id  = releases.rel_id
and rel_name = 'Mistral v3.4.8'
group by releases.rel_name, releases.rel_id

The correct result from QC is 489 (see screenshot in attachment).

What I forgot ? 

Thanks in advance !