My guess: it's more of a "don't make me think" type thing.
Can you divide 9.99 by 12 in your head? How long would it take? How sure would you be of the answer after 1, 2, 3 ... 5 seconds?
In that impulsive fraction of a second that you have the user about to click the button and they are battling within themselves if they can do the calculation then it increases the chances that the "buy" side will win. Being able to do the calculation easily probably also gives a reflection of friendliness and ease onto the entire rest of the service as well.
Well personally it would take me a couple of seconds to get to "roughly 85c/month", but I wouldn't actually be thinking that when deciding to signup.
However, if I did care about how much $9.99/year is each month, rather than even trying to be slightly accurate, I'd round it off as "less than a dollar a month" and go straight ahead. That extra 17c/month doesn't make a difference to me.
(Obviously both my calculations speed and the amount I care about price differences when talking about that amount of money isn't neccesarily equal to that of other, possibly most other, potential customers.)
Can you divide 9.99 by 12 in your head? How long would it take? How sure would you be of the answer after 1, 2, 3 ... 5 seconds?
In that impulsive fraction of a second that you have the user about to click the button and they are battling within themselves if they can do the calculation then it increases the chances that the "buy" side will win. Being able to do the calculation easily probably also gives a reflection of friendliness and ease onto the entire rest of the service as well.