The PAYG plan in meant to be used by companies that don't book meetings too far ahead in the future, as this will increase their costs.
your monthly billing day is the 21st.
on March 21st 2019 you receive an invoice for the time interval between February 21st and March 20th
the invoice shows you were booked for 300 meetings
Reports show your company booked only 150 meetings in that time interval
The reason this happens is because you are not charged only for the meetings scheduled in that interval, but for all meetings scheduled for dates after February 21st (the start of the invoiced interval).
This means that if on March 10th you book a meeting for June 10th, this meeting will be charged on the invoices issued on March 21st, April 21st and May 21st and June 21st.