Odd sum() problem in 7.2.2

I'm stuck. The following two snippets gives me two different results,
and I can't figure out why;

1) select abs(0 + int4(sum((int4(ss7_rel_timestamp_sec) - int4
   (start_timestamp_sec)) / 60))) from session_billed where (b_nr
   like '12345678%' and datetime(ss7_rel_timestamp_sec) >=
   '2002-09-01' and datetime(ss7_rel_timestamp_sec) < '2002-10-01'
   and session_status_reached >= 20);

   = 5845507 and count(*) [1] gives me: 286109

2) select int4(sum(ss7_rel_timestamp_sec - start_timestamp_sec) / 60)
   from session_billed where b_nr like '12345678%' and
   ss7_rel_timestamp_sec >= '2002-09-01' and ss7_rel_timestamp_sec
   < '2002-10-01' and session_status_reached >= 20;

   = 5986420 and count(*) [1] gives me: 286304

 ss7_rel_timestamp_sec = abstime
   start_timestamp_sec = abstime
                  b_nr = character varying(32)
session_status_reached = integer

There have been no changes in the table between these two queries.
PostgreSQL 7.2.2 on i686-pc-linux-gnu, compiled by GCC 2.95.4.

[1] select count(*) from session_billed where <rest of original query>

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