SQL & PL/SQL :: %ROWCOUNT Returning Incorrect Value For Updates?

Consider the statement below:

Update employee e
set e.dept_id = (select d.dept_id
from dept d

[Code].....

The above is not the exact code which I am executing but an exact replica of the logic implied in my code.

Now, when i display the value of 'rows_updated' it returns a value greater than 0,i.e 3 but it should ideally return 0
since there are no records matching for the condition:
(select d.dept_id
from dept d
where e.dept_name = d.dept_name)

So, I executed the statement:
select count(*) from employee e
where emp_id = 1234
and exists
(select 1
from emp_his ee
where e.emp_id = ee.emp_id)
and the result was 3 which is the same value returned by %rowcount.

why this is happening as I am getting incorrect values in %rowcount for the number of rows updated.