http://www.juniper.net/documentation/en_US/junos13.3/topics/concept/policy-per-packet-load-balancing-overview.html
http://www.juniper.net/techpubs/en_US/junos12.3/topics/concept/policy-per-packet-load-balancing-overview.html
http://www.juniper.net/techpubs/en_US/junos12.3/topics/usage-guidelines/policy-configuring-per-packet-load-balancing.html
These articles should help explain in details. Not sure we are understanding what you do not understand about load balancing. You have configured it and it is working as designed.