Is the reason that $12 converted better than $9.99 definitely because users are able to visualise it as one dollar a month? Maybe people see the .99 and are trained to think "99 cents is a marketing plan to make me think it's cheaper than it is", thus causing it to have the opposite effect? Maybe people see $9.99 as a generic price, implying it was picked fairly randomly, whereas $12 is a slightly odd number to chose…
Seems to be intuitive enough to happen subconsciously.
The other price points just appear as one fixed sum in the mind, which is much less likely to be spent on a whim.