Ubuntu PXE boot mount wrong NFS share

This question may be simple but I couldn't figure out and was stuck here for the whole day already. Any suggestions are appreciated.

I have two boxes, one as the server and other one running as the client.

Server running Ubuntu 11.04, root fs on LVM2 logical volume, static IP 192.168.2.20. After installing the clean system I copied the complete root partition to another LV, say /dev/data/root. All files are under /nfsroot folder on that LV. I made two snapshot of /dev/data/root (with lvcreate -s) as /dev/data/master and /dev/data/node1. I mounted these two LVs on the server as /mnt/{master,node1} and created both /mnt/master/nfsroot/master and /mnt/node1/nfsroot/node1 files so I won't get lost. Then I shared both two mounts via NFS (nfs-kernel-server) as

/mnt/master/nfsroot     192.168.2.0/24(rw,no_root_squash,async,insecure,no_subtree_check)
/mnt/node1/nfsroot      192.168.2.0/24(rw,no_root_squash,async,insecure,no_subtree_check)

I mounted these NFS shares and saw the right files in the right place.

I then installed other components for networking boot following Diskless Ubuntu Howto https://help.ubuntu.com/community/DisklessUbuntuHowto. My PXElinux config for box 2 is

DEFAULT linux

LABEL linux
KERNEL vmlinuz-2.6.38-8-server
APPEND root=/dev/nfs initrd=initrd.img-2.6.38-8-server nfsroot=192.168.2.20:/mnt/node1/nfsroot ip=dhcp rw

Box 2 was boot successfully but when I looked at / I saw /master file. I checked /proc/cmdline and nfsroot was

nfsroot=192.168.2.20:/mnt/node1/nfsroot

and mount | grep ' / ' also shows 192.168.2.20:/mnt/node1/nfsroot

I then mount both shared on box 2 under /mnt/{master,node1}. Even when mounting different server shares, two filesystems appears to be mapped to the same (master) share on the server.

Any one have any ideas why?

For a image of screen shot describing the problem please see http://ww3.sinaimg.cn/large/64b3c41egw1do3ftklgshj.jpg