BUG #2117: inconsistency in sum

The following bug has been logged online:

Bug reference:      2117
Logged by:          Adam Kolany
Email address:      dr.a.kolany@wp.pl
PostgreSQL version: 8.0.1
Operating system:   Linux, SuSe 9.2
Description:        inconsistency in sum
Details:

can you explain this?
======================================================
stoff=> select sum(wplyw),sum(wydatek), sum(wplyw)-sum(wydatek) from
rachunki where okres<6;
  sum  |   sum   | ?column?
-------+---------+----------
 73745 | 6712.55 |  67032.5
(1 row)

stoff=>
======================================================

in real, the 'sum(wplyw)' should be:  73745,05