I have been reading Bulletproof SSL and TLS
Diffie Hellman is a key exchange mechanism, in which each of server and client share secret i.e. gx and gy respectively with each other where g is generator of some group. They calculate secret S = (gy)x and (gx)y respectively. Note that an attacker in the middle cannot get the value of S from gx and gy, since discrete logarithm is a hard problem. But, an active attacker can masquerade and fool both server and client.
Now in RSA_DH, the value which server sends to client i.e. gx is fixed and stored in certificate. So, the "ServerKeyExchange" message is not sent. Now, client sends its gy value in "ClientKeyExchange" message and this is encrypted with RSA public key cryptography algorithm whose certificate server has already sent in "Certificate" message.
In RSA_DHE, the "ServerKeyExchange" message takes place where server sends gx to the client.
Now my question is what is the difference between the two? Since the premaster secret is calculated from the value (gx)y, Therefore, even in RSA_DH, if the client sends different gy values in each session, then (gx)y value will be different. (I think even if he doesn't still my argument holds) Therefore it will achieve forward secrecy as in RSA_DHE. And the poor attacker won't be able to get premaster secret even after getting the value of gy by compromising the server private key. So, in both cases he won't be able to calculate premaster secret. So why Ephemeral one (RSA_DHE) is said to achieve forward secrecy and other one not?