"rotate_limit" seems not to work

[chuenlye]

My config:
$ cat ~/.bu2sw.conf | tail -n 3
[backup]
rotate_limit: 10

My cron:
0 4 * * * root cd /var/lib/ganglia/; tar cf - * | lzop | cat > /var/tmp/rrd_day_bk/ganglia_rrds.lzop; bu2sw --container ganglia-a4c -p /var/tmp/rrd_day_bk/ganglia_rrds.lzop

But I found 470 backups!
$ bu2sw --container ganglia-a4c -l | grep ganglia_rrds.lzop -c
470

Is there something wrong?