Annual prepaid v. Annual paid monthly

I am looking to buy access to CC, but am confused. Usually any prepaid plan will be cheaper than a pay monthly plan but not in the case of the Student version annual subs. This works out at about £1.50 cheaper to pay monthly (annual contract) than to prepay for the year. Not much, I know,  but every little helps! 

Is this a mistake? It's not enough to be sweetener towards one method or the other.