As per OpenStack Networking (neutron) API v2.0, "subnet_id" is an attribute of LoadBalancerPool, whereas in ODL it is an attribute of member of the pool (NeutronLoadBalancerPoolMembersNorthbound).

Creating LoadBalancerPool through openstack UI
openstack will provide a subnet_id while creating a LoadBalancerPool.
whereas, when LoadBalancerPool creation call reaches ODL, the NeutronLoadBalancerPool object does not have "subnet_id" as an attribute which can be mapped to the one supplied by openstack.