Hello Giusy Castiglione,
I think you are meeting unit conversion issue.
Notes 393629, 525583 and 980466 are valid.
Please check T-code: OPK4
If you select "Automatic Optimization" (or "Default time" in releases lower than Release 4.6), the system chooses a unit that has a rounding error of less than 1/1000 for the value instead of a predefined unit.
Hope this information helps.
Best Regards,
Owen