Acct-Unique-Session-Id not unique!

[Vega Freeradius]

Hi!
Forgive me for by bad English.
I've some problem with acctuniqueid. I use two freeradius server with 
two node mysql cluster.

My freeradius version is "2.0.5". On both server threre is the same config.
The modules/acct_unique content  on both server is  set to default:
 
acct_unique {
        key = "User-Name, Acct-Session-Id, NAS-IP-Address, 
Client-IP-Address, NAS-Port"
}



But servers generate two different "uniqueid" from the same data!

this is the START record (issued by first server)

Thu Nov 26 15:00:45 2009
        Acct-Status-Type = Start
        User-Name = "studioss at d.gbnetc.it"
        NAS-Identifier = "r-nl035"
        Acct-Session-Id = "erx atm 1/3.10083:1.83:0148107119"
        NAS-IP-Address = 217.141.253.49
        Framed-IP-Address = 79.34.44.195
        NAS-Port = 318832723
        NAS-Port-Id = "atm 1/3.10083:1.83"
        Acct-Unique-Session-Id = "40f5c1e75a777864"

this is the STOP record (issued by second server)


Thu Nov 26 15:39:00 2009
        Acct-Status-Type = Stop
        User-Name = "studioss at d.gbnetc.it"
        NAS-Identifier = "r-nl035"
        Acct-Session-Id = "erx atm 1/3.10083:1.83:0148107119"
        NAS-IP-Address = 217.141.253.49
        Framed-IP-Address = 79.34.44.195
        NAS-Port = 318832723
        NAS-Port-Id = "atm 1/3.10083:1.83"  
        Acct-Unique-Session-Id = "fb963455e99b780c"

I delete some data before post it here. I see that
User-Name, Acct-Session-Id, NAS-IP-Address, Client-IP-Address, NAS-Port
ar the same on both record. But Acct-Unique-Session-Id is different!!!! 
How this could be ?

Thanks in advance

Marco