According to the talent calculator (http://us.battle.net/wow/en/tool/talent-calculator), Atonement is supposed to be 38, and Archangel is supposed to be 50. It's normally been well out of date, but perhaps it's up to the minute for once!
getting atonement earlier is fine, u can literally just dps in dungeons to make everything go that little bit faster, while leveling my priest just before MoP release I was topping meters all the way into cata content =D
Leveling my second priest as disc through dungeons, and got Atonement both times at lvl 38. Asked about this on EU forums, but got no reply from a "blue", so i hope that's a tooltip bug.