http://www.newsweek.com/pokemon-go-cp-update-changes-1179868

Niantic informed Pokémon Go players that some changes will be coming to the mobile game including how the weather will change and the combat power (CP) of certain Pokémon will be rebalanced.

While the weather changes have yet to be noticed, the CP of Pokémon in Pokémon Go were changed and then suddenly reverted back by Niantic. Some speculate Niantic reverted the CP changes to fix a bug that changed some Pokémon stats like HP.