Networks are constantly changing and evolving some hosts allow equal outbound policy while inbound policy is set to the cheapest network link (great for absorbing DDOS without getting huge bills) some allow equal inbound policy. Many adjust policy as necessary, mainly outbound path selection, we have very little control over inbound policy, upstreams can strip our prepending or ignore it completely. I've seen traffic cross Level3 and they dump it to Cogent despite a connection with Level3 and the same network pathing. Level3 just determined pushing traffic out that link was in their best interests.