UNIX df command

I have the Linux server:
$uname -a
Linux server01 2.6.18-194.3.1.el5 #1 SMP Thu May 13 13:08:30 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
#cd /var/FTP/MY_DATA
server01:/var/FTP/MY_DATA# df -k
/dev/sda2             38654768  14706932  21952564  41% /
/dev/sda3              8123200   2447272   5256632  32% /var
/dev/sda1               147764     28986    111149  21% /boot
tmpfs                  1221248         0   1221248   0% /dev/shm
server01:/var/FTP/MY_DATA# df -k .
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda3              8123200   2447080   5256632  32% /var
Now I create a directory named 1:
server01:/var/FTP/MY_DATA# mkdir 1
server01:/var/FTP/MY_DATA#cd 1
server01:/var/FTP/MY_DATA/1#
I copied some files into the directory above. Now:
server01:/var/FTP/MY_DATA/1# ls -l
-rw-r--r-- 1 root other  177532 Jan  4 10:24 1357257600.myfile3.dat.bz2
-rw-r--r-- 1 root other  168743 Jan  4 10:24 1357257600.myfile2.dat.bz2
-rw-r--r-- 1 root other   87845 Jan  4 10:24 1357257600.myfile1.dat.bz2
-rw-r--r-- 1 root other 1510618 Jan  4 10:26 1357257600.myfile1.dat
-rw-r--r-- 1 root other 3788740 Jan  4 10:26 1357257600.myfile2.dat
-rw-r--r-- 1 root other 5700447 Jan  4 10:26 1357257600.myfile3.dat
server01:/var/FTP/MY_DATA/1#df -k .
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda3              8123200   2458704   5245200  32% /var

I do not understand how it shows the available size under /var/FTP/MY_DATA/ (or under /var).
Before copying the files, it said 5256632KB is available (5.25GB is available).
After copying, it says 5245200KB available (5.24GB available).
But the copied files are larger than the available space there. For ex, 1357257600.myfile3.dat is 5700447KB (5.7GB).
Could you let me know where I am making mistake?
Actually problem occurs when an automated process tries to copy the above 6 files onto the folder above and gets the error with this error details:
"un.net.ftp.FtpProtocolException: 553 Could not create file." Apparently from insufficient space issue.

Could you help me how I could interpret the error and the viable space on the OS directory?
Thank you.