"It remains an interesting study into just how these "errors" due complexity of the payment structures always results in underpayment."
When working in a large multi-national company I once put a similar question to a (relatively) friendly face in HR.
HR & Finance are judged as everyone else on "performance" - a major item is sticking to (or coming in under) budget. Thus in any situation that allows for choice it is very tempting to go the cheapest route - this may not be a major decision on that item but it may gain you some shuffle space if something else in the future unexpectedly goes wrong. The absolute maximum you can expect is what is in the policy - and normally you will get (a lot or a little) less depending on how many "decisions" are required.
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It's not nice but it's human nature in a system that measures people by "metrics" and "output".