T&C change on a final bill

I am guessing it was needed to match T&C with BT or so. Though, as bills are per calendar month (start month is pro rata) then what is the difference with start month and end month?
Please bear in mind that house/flat rentals are usually not in caledar months so almost always it will in cuckoo’s favour.
In a spirit of transparency, possibly it would be good if some further details are given to retain the trust and engagement.