I think the logic is becoming more easy to understand. If your booked cabin is expected to be oversold, and there is room in a higher cabin, your OT upgrade will clear.

If your cabin is not oversold, your OT upgrade will not clear no matter how many seats are available in higher cabins.

Then you can argue that this is not sensible, but anyway this seems to be the logic of OT upgrades.

So always have some points ready for those situations when OT upgrades don't clear.