When you reach a new commission tier, the new percentage is not backdated. It will come into effect two months after you reach it, and apply to all bookings made that month.
If you have 60 confirmed bookings in January, the new bracket will come into effect in March. This means that all bookings made in March will fall under this higher commission rate.
Commission is always paid based on the commission tier you were in at the time the guest made the booking.
The chart below shows when commission rates are adjusted and how they affect bookings: