Why ORA-01401: inserted value too large for column on 10g and not on 9i?

We do the same import in an Oracle9i-database and oracle 10g, when we do the import in 10g we encounter this famous error ORA-01401: inserted value too large for column , where it stated for instance that the value too be inserted had a length of 21 characters where the original table-definition on source and destination had a type varchar2(20 byte) and columns in the source-database don't have values longer than 20 chars, which should also be rather impossible.
The workaround now was on the destination database (with National charset UTF8) create the tables prior to import and change the columntype from varchar2(20 byte) in varchar2(20 char). The weird thing is that import in an Oracle 9i-database with nchar UTF8, where the source is the same 10g with nchar AL16UTF16, works fine without workarounds.