IP load sharing kicks in when IP route table contains more than one path to a destination and the paths each have the lowest cost. IP load sharing uses a hashing algorithm based on the source IP address, destination IP address, and protocol field in the IP header, TCP, and UDP information to distribute the traffic. In BGP, When multiple paths to the same destination is recieved, BGP selects the best path based on a list of attributes. This is to have granular control over how you forward your traffic and to which autonomous systems instead of just going for the shortest path based on a metric. Only the best path gets into the routing table and remaining will show up in the bgp table.
The best path is selected based on following attributes and priority.
Priority Attribute 1 Weight 2 Local Preference 3 Originate 4 AS path length 5 Origin code 6 MED 7 eBGP path over iBGP path 8 Shortest IGP path to BGP next hop 9 Oldest path 10 Router ID 11 Neighbor IP address