Google is constantly working to improve our systems that protect users from Potentially Harmful Applications (PHAs). Usually, PHA authors attempt to install their harmful apps on as many devices as possible. However, a few PHA authors spend substantial effort, time, and money to create and install their harmful app on one or a very small number of devices. This is known as a targeted attack.

https://security.googleblog.com/2017/04/an-investigation-of-chrysaor-malware-on.html