help:about ftplib?

[Old gun]

my system is 
    mandrake linux 8.1 and python 2.1.1

when i import ftplib and ftp.retrbinary("RETR filename",...,...)
but i run "ls -l filename"
this file's size is 0 byte.
after a moment,its size is normal.

and on windows 98 i use ftplib in python 2.1,there is no thus problem.

why?