Why need two SubmitWindowedPoSt messages in one deadline?

[sokach-dev]

I found in the log that, once again, miner sent two messages in the windowpost message submission. Why is that?

https://filfox.info/en/message/bafy2bzaceaaugr7dszmcute2u4uhwokmjpmvtvs4rsh3nzc7eff74uwrvodfk?t=1

https://filfox.info/en/message/bafy2bzacedfxofeeypl3cq2xfegnzgvvwnn7i4qlryuosw4m7ejeudp4ednsg?t=1

[TippyFlitsUK]

This explains why - filecoin-project/builtin-actors#1436