Comment by phrotoma

I suspect it’s because random assignment is simpler to implement than incremental round robin when the targets are moving rapidly (eg. pods coming and going in K8s). If you are iterating over N replicas in true round-robin fashion and the nth+1 replica comes online, you need to manage inserting it into your rotation to maintain true incremental round robin. To do it probabilistically you just update your weights. This is how kube-proxy sets up iptables based load balancing.