DHCP on NW5.1 SP5 :

let's say I have two segment :
A) (pool from -> on ServerA
(site A)
B) (pool from -> on ServerB
(site B)
a user A (from site A)is defined in pool A to receive an address
"assignment type :Manual, Mac Type: 1 Ethernet 10", everything is ok,
When the user move to site B, the user does not receive any IP
until I create a manual assignemt for that Mac Address in that pool.
Even if forcing with "IPCONFIG /RELEASE /RENEW /PLEASE" ... etc

why ?
what did I did wrong ?