I was looking last month when I was closing on on the threshold and it said last updated October 4th. Then on about November 7-8 it updated and said as of November 4th (it still says that). This is in alignment with my statement dates, so I'm assuming the information updates at the same time as the miles are deposited into your account.
Of course this doesn't explain you receiving two updates in the same month, so perhaps there is an additional trigger; or perhaps they are updating more frequently to encourage you to hit the threshold.