Premium support and or enterprise Support is a charge for support charges that AWS adds usually between the 13th-16th of the month.
When looking into a specific month before those dates those charges will not appear as they were not charged yet.
The confusion comes from the fact that although those charges are added between the 13th-16th of the month they are registered under the 1st of the month.
This could be the reason why a customer that sees premium support charges every month on the 1st, does not see them in a specific month before the 13th -16th.