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, implying that it was carefully picked as a price that gives users good value? (It would have that effect on me - $9.99 suggests they haven't worked out how much they need to charge me to make a profit, and just picked a rounded number that looks normal.)
I'd like to see $12 compared to $14, compared to $11.99, compared to $24, compared to $20.