simpleERB calculates your covers at the point of booking.
So if I make a booking today for some future date later this year, it will be counted in this current period, even though the booking is in a future month.
Conversely, if you have bookings in your diary for, say, the festive period - that were booked earlier in the year. These do not count against your December allowance.
If the booking is cancelled or deleted, then the covers for that booking are set to zero, and then monthly totals are recalculated. This means that a cancelled booking does not count against your monthly covers.