Unable to delete a file in linux

On a fine day, I found a normal looking file, which I just wanted to clean.

$ ls -l var/t/log/tlog

-rwxrwxrwx 1 root root 2770 Apr 6 14:05 var/t/log/tlog

$ sudo rm -rf var/t/log/tlog

rm: cannot remove `var/t/log/tlog': Operation not permitted

$ sudo rm -rf var/t/log

rm: cannot remove `var/t/log/tlog': Operation not permitted

$ sudo getfacl var/t/log/tlog

# file: var/t/log/tlog

# owner: root

# group: root

user::rwx

group::rwx

other::rwx

then why the hello even if I am root, I cannot delete the file.

#cd var/t/log

# lsattr tlog

-----a------- tlog

# chattr -a tlog

# lsattr tlog

------------- tlog

# rm -rf yinstlog

Done.