In GuestCentrix when a reservation is made, it is recorded in your database as a ‘booking’.
This includes groups, mini-groups and FIT’s.
This record includes the room rate, the tax on the room rate and details of the booking including number of guests, special requests etc.
The guest’s bill (or folio) is recorded as all transactions that are linked to that folio.  Separately to the booking.
An availability forecast report (or any forecast report) looks at the booking records for the amount of time you have selected to report on. It adds up all the rooms booked, and the total rates etc.  This report is not able to calculate differences in revenue that have been manually posted. It will be accurate for forward bookings however, for past dates the forecast reports are best used as an indication of revenue, not actual revenues. 
A statistics report calculates all the postings on each folio based on the transactions that have been posted into the guests account. This will give 100% accurate information for any past dates and is not able to give a forecast of revenue. This may be different to your forecast report due to manual postings and adjustments such as the guest room rate rolled incorrectly and a manual adjustment was made or travel agent commission has been removed from the accommodation revenue.
Other examples might be manual postings for roll away beds, voided accommodation charges an so on.
What is the best GuestCentrix report to use for forecasting?  The Availability forecast report
What is the best GuestCentrix report to check your statistics? The Monthly Statistics or Monthly Income Summary report