That's the way pay in advance subscriptions work, a major saving for the purchaser in exchange for commitment to a number of issues.
In the case of ME the benefit of an annual subscription is particularly generous. The 12 issues of the 26 you have already received have a cover cost of cost slightly more (£59) than an annual direct debit subscription (£58).
Obviously, this reduction only applies if you commit to a full year's subscription.
Neil.