There are 2 key differences in the way Flurry and Google Analytics report "New Users".
1) Flurry looks at unique devices, and not user IDs. That means that if a user uses the same user ID to install the app on multiple devices, Google will count it as 1 Sale, whereas Flurry will count it as multiple "New Users".
2) Flurry counts a user only when they have launched the app, not upon install. Which means that if Google Analytics report an "install" today, but the user launches the app tomorrow, Flurry system will count it as a New User tomorrow.
Full disclosure: I work at Flurry