The Ran there suggests that by Kedushin the guy takes his own word more serious, and therefore it is final. Rashi there does seem to imply that there is an inherent difference. Otherwise, the Ran and Rosh express their wonder at the inconsistency.
I wonder, though, why the Machlokes of Reb Meir and Reb Yosi, regarding Tafus Lashon Rishon or Achron, is not brought up. Also the Machlokes Beis Shamai and Beis Hillel in Nazir. Where does all this come to play? (Now I have how to keep myself busy)