Source IP address field in RREP on DSR routing

[acu281]

Hey guys
I have a question about routing in MANET using Dynamic Source Routing protocol.
IN RFC4728 (DSR) in section "IP fields" of RREP (Route Reply) packet we have this:

Source Address
Set to the address of the node sending the Route Reply. In the
case of a node sending a reply from its Route Cache (Section
3.3.2) or sending a gratuitous Route Reply (Section 3.4.3),
this address can differ from the address that was the target of
the Route Discovery.

ok.
I read in several books and also in rfc4728 that: when a source node (node that initiate route discovery process) receive a RREP it are not able to understad which node sent that RREP!!
But why?
rfc say that the value of source IP address field of RREP packet is IP address of node that send RREP. and this packet forwarded toward Source node.
My question is why??! why source node can not always understand that which node is sender of RREP?

Please guide me
tnx