firebird 2.5.0.26074 on windows 7 home premium x64

Below is a (manually obscured, simplified table) scenario that (hopefully) matches what I'm doing, with actual return count values.

Is my SQL just woefully incorrect, or am I possibly experiencing a problem with fb 2.5, in apparently seeing a great many more records deleted than I expect, based on the various queries.

I expected the last query to yield (123686 - 7617) = 116069, not the reported 107548.

Why didn't it???

data content notes:
1)all form_id values in table1 are unique
2)all subform_id values in table2 are unique EXCEPT there are many duplicates of '0' (zero), _and_, some of those _are_ matched to table1 entities that are like '%hello%' (i.e. t1.form_id = t2.form_id and t2.subform_id = 0) - I don't currently know counts as I've been waiting for an ISQL query (because I want to see what I entered) I did botch, to finish (and Ctrl-C didn't interrupt it.)


I will attempt to provide additional information, if what I have presented is insufficient for analysis.

TIA for any assistance.

==============================================
create table table1
(form_id as Numeric
,form_filename_src varchar(256)
) ;

create index idx_table1 on table1(form_id) ;

create table table2
(subform_id as numeric
,form_id as numeric
) ;

create index idxA_table2 on table2(form_id) ;
create index idxB_table2 on table2(subform_id) ;

select count(form_filename_src) from table1 where form_filename_src like
'%hello%' ;

7617

select count(subform_id) from (select t2.subform_id from table2 t2, table1
t1 where t1.form_id = t2.form_id and t1.form_filename_src like '%hello%') ;

7617

select count(*) from table2 ;

123686

delete from table2 where subform_id in (select t2.subform_id from table2 t2,
table1 t1 where t2.form_id = t1.form_id and t1.form_filename_src like
'%hello%') ;

(no delete count or rows affected returned)

select count(*) from table2 ;

107548

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