Xbee sensor sleep mode appears inconsistent

Hi
I am using the Xbee sensors ZB bat/L/T/H and a ConnectPort X2. The sensors are set for extended sleep and should wake up and send a single packet to the controller every 2mins which is running a single script to read that packet and post to a remote sql server.

When the program 1st sees a sensor it sets the following
‘ir’, 1700)
‘ST’, 2000)
‘SP’, 2000)
‘SN’, 6)
‘SO’, 4)
‘dh’, 0)
‘dl’, 0)
‘AC’, ‘’)

the xbee netowrk is opened with

xbee_socket = socket.socket(socket.AF_ZIGBEE,socket.SOCK_DGRAM,socket.ZBS_PROT_TRANSPORT) 
xbee_socket.bind(('', 0xe8, 0, 0))
xbee_socket.settimeout(1)

and accessed

nodes_list = zigbee.getnodelist(False)      

If I see any nodes in nodes_list I issue

packet, source = xbee_socket.recvfrom(72)

and then
if (source[3] == 0x92)

I process the packet
I end up getting regular updates to my sql table but some outside the 2mins period expected.

device_id device_time reading Mvolt rssi rec_nos
[00:13:a2:00:40:79:2d:db] 2013-02-12T19:18:00 75.02 2927 35 15052
[00:13:a2:00:40:79:2d:db] 2013-02-12T19:15:59 75.02 2927 35 15051
[00:13:a2:00:40:79:2d:db] 2013-02-12T19:13:59 75.02 2927 35 15050
[00:13:a2:00:40:79:2d:db] 2013-02-12T19:06:02 < 75.02 2926 35 15049
[00:13:a2:00:40:79:2d:db] 2013-02-12T18:58:06 75.02 2927 35 15048
[00:13:a2:00:40:79:2d:db] 2013-02-12T18:56:07 < 75.02 2927 35 15047
[00:13:a2:00:40:79:2d:db] 2013-02-12T18:48:10 < 75.02 2927 35 15046

Does anyone have any suggestions why that is or what it is I am missing here?
Thanks for any help
CT