Why length()/dump() return different substring length values ??

[KENNY L. CHEN]

I have a table with a CHAR column which stored a COBOL redefined
record.
I tried to retrieve part of the field from the column with Oracle
substr function.

Here is one of example: The SQL/result are shown below:
SQL:
select
length(substr(MY_COLUMN,36,8)) str_length,
dump(substr(MY_COLUMN,36,8)) INFO,
rawtohex(substr(MY_COLUMN,36,8)) raw_value
from TABLEX
Result:
str_length:8
INFO :Typ=1 Len=10: 0,0,0,0,7,151,237,139,70,89
rawtohex :000000000797ED8B4659

The value returns by length function is 8 which is what I expected.
The value returns by dump function is with LEN=10 is not what I
expected.

Can anyone can explain why the two Oracle functions (length() and
dump()) return different length values for the same field ?

THANKS

KEN