This situation may be when in some tables somebody declared field as VARCHAR2 and it works when CHARACTER SET has 1 byte and you have full filling fields.
When you (or exp/imp utility) insert into CHAR or VARCHAR2 fileds in full filling field (European or Asian lang) length of field may be increase (more then field lenght).
REFERENCE about UTF-8 Encoding
UTF-8 is the 8-bit encoding of Unicode. It is a variable-width encoding in which the character codes 0x00 through 0x7F have the same meaning as ASCII. One Unicode
character can be 1 byte, 2 bytes, 3 bytes or 4 bytes in this encoding. Characters from the European scripts are represented in either 1 or 2 bytes.