Note: If the matching rule number (3 for prerouting chain of raw table) is greater than the number of rules there, it means there wasn't a match for any of the rules and it is just returning to whomever called the chain.
came to this question too and dit some testing: policy:5 seem to refer to the fictive last "rule" of your chain, where the policy is considered. so loonyuni's traced chain should have 4 explicit rules in it.