This extra is both oracle rewards and community pool which were derived from Luna burned when UST minted. (Seignorage).
See: Proposal to burn all seigniorage 🔥 - #48 by WMoon
The community pool was once sent to a burn wallet, and this was supposed to happen again via proposal (and was what was voted via governance). But this hasn’t happened and as you can see from my posts on link no-one answers this.
I think the plan is now to use the comm pool Luna for insurance for deposits on anchor. But my own view is that what is currently in the comm pool should be burned and if insurance pool is to be generated it should be from future community pool funds. As it will continue to grow so long as UST demand increases… Hopefully will get a response on this? Thanks WMoon.