Being charged pro rata to change a debit order date

I called in to have my debit order date changed from the 1st of each month to the 26th of each month,meaning i am paying earlier.never in my life have i heard of pro rata being charged to change the date,why should i pay extra( pro rata to have the date changed),its a mere change of date,not adding anything additional to my policy,its an exiting policy . I am changing all my debit orders to run earlier with no hassles,only outsurance and their ridiculous arrangements to pay pro rata.Get your act together outsurance as i consider changing insurance should this be the type or service/daylight roberry i am faced with