Cisco :: Why 2 Encryption Keys For IPSEC VPN

how IPSEC VPN works but i hit a stumbling block understanding symmetric encryption keys.Here is my understanding about the process

1.Peers will negotiate plocies

2.Authenticate using pre-shared or certificates

3.Exchange DH Public Keys

4.Using Public keys encrypt symmetric key and exchange the same key which will be useful for communication

5.maintain sessions

But when we are configuring we will define encryption keys in isakmp phase and ipsec transform set ,i thought we will use the same encryption key for both management and data communication in fact i thought management phase is to give us a securely exchanged encryption key for the data tunnel.But we can use 2 different encryption keys in 2 phase i am bit confused.