Following the announcement that trainers will be getting double XP and stardust during Thanksgiving week, Niantic announced that several Pokemon got CP balance adjustments to make battling in Gyms more competitive.
Here the official announcement:
We have made adjustments to the CP (Combat Power) of various Pokémon. Some Pokémon, such as Alakazam, Rhydon, and Gengar will have their CP increased. Others will have their CP lowered. These changes will allow a more balanced and competitive battling and training experience in Gyms. We will continue to adjust the CP going forward to improve the game balance when necessary.”
See buffed & nerfed Pokemon below