problem: sum is incorrect (e-10)

Hi all,
I've a problem.

The query select mo_qta from oobjm.movoptittraded where mo_opzione =
'1,253CEUUS16M08' gives:
MO_QTA
------------------------
  +2.50347710000000E+005
  +2.50347710000000E+005
  +2.50347710000000E+005
  +3.33796940000000E+005
  +1.00139082000000E+006
  -2.50347710000000E+005
  -2.50347710000000E+005
  -2.50347710000000E+005
  -3.33796940000000E+005
  -1.00139082000000E+006

and the query select sum(mo_qta) from oobjm.movoptittraded where
mo_opzione = '1,253CEUUS16M08' gives:
1
------------------------
  +3.49245965480804E-010

The field is a double.

I think is impossible...
How can i find where the problem is?

Thanks all,
DM