RSSI analysis of 3 zigbees network.

XBee & XBee Gateway XBee - 802.15.4
1

I’m having a problem ATND command to know RSSI.
I have 3 zigbees network with the Coordinator sending packets to one router, the router sending it to end device and end device send the packets back to the coordinator. I need to know the RSSI of this network. I’m using this code:

# RSSI

tty = serial.Serial('/dev/ttyUSB0',timeout=4)

outb = '+++'
tty.write(outb)
inb = tty.read(2)
print inb

loop = 2
if inb == 'OK' :
        outb = 'ATND'+chr(0x0d)
        tty.write(outb)
        for i in range(0,loop) :
                inb = tty.read(24)
		print 'inb:'
		print inb
                print inb[2:9]+' '+inb[10:18]
                print '-'+str(int(inb[19]+inb[20],16))+'dBm'
		print ' '
		log.write('-'+str(int(inb[19]+inb[20],16))+'
')

But when i run this code two times i have different RSSI values.
There are the results:
inb:
EN6E7F54
13A200 406E7F54
-36dBm
inb:
4076F0CE
13A200
-64dBm

Running the program again i got:
inb:
RO76F6CB
13A200 4076F6CB
-36dBm

inb:
406E7F54
406200
-127dBm

Another time:
inb:
EN6E7F54
13A200 406E7F54
-41dBm

inb:
4076F0CE
13A200
-64dBm

Why am i having differente values to the RSSI?