p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.
because the annual plan paid monthly is less costly (per month) than the corresponding monthly plan (if there is one). that lesser cost is offset by the 1 year commitment required for the annual plan and that 1 year commitment is enforced by a penalty (= 1/2 the remaining cost) if the annual plan is cancelled before the 1 year contract/commitment.