Can someone explain to me why this is happening

Question

Ok I am trying to add two values in an update statement. Part of my code is as follows:
update account a
set a.total_term_births =( SELECT b.total_term_births
FROM ACCOUNT b
WHERE b.account_id = 98778
+
a.total_term_births )
where a.account_id = 98777
The answer I am expecting is 5244, but
SQL> select total_term_births from account where account_id = 98777
2
SQL> /
TOTAL_TERM_BIRTHS
268
Now, if I change the above coorelated sub query to as follows:
update account a
set a.total_term_births = ( a.total_term_births + (
SELECT b.total_term_births
FROM ACCOUNT b
WHERE b.account_id = 98778 ))
where a.account_id = 98777
I get 5244 the correct answer. In my mind both syntaxes are correct.
Am I right in my assumptions.

Answer 1

You are not getting the expected results with the first version because the subquery is looking for an account_id value of (98778 + the_value_of_a.total_term_births). So, if row 1 had a total_term_births value of 123, the correlated subquery would be looking for an account_id of 98901, and if row 2 had a value of 50, the subquery would be looking for an account_id of 98828, and so on. This is not what you want at all.
The second version is the correct version of the query.