We've got some breaking news for you: Google has renamed its Keep app to 'Keep Notes.' We know, it's mind-boggling stuff. Nothing in the app seems to have changed otherwise, though, so it looks like someone at Google just got bored and decided to add another word to this app name.
As for why exactly 'Notes' was added to the app name, we're not sure. It does better state the app's purpose, but it's also not as simplistic as the one-word 'Keep.' The web version still simply says 'Google Keep,' for what it's worth.