Along with the longstanding ability to search your web history and contacts, Google also indexes some third-party apps and takes you directly into that application if it’s a relevant search result.
However, you can control which apps and services on your device are part of the search index. To get there, launch Google Now and tap the hamburger menu (three horizontal lines) and choose Phone Search.
Then click the boxes to specify which apps you want included or left out. If there’s a Google app you don’t use that much, in my case Google Play Books (I use Kindle), it’s to your advantage to turn that off. This way those auto-suggestions won’t take up precious screen real estate in search results.
There are over 70 different apps that hook in to Google Now. As you add more of them, you can check back here in the Phone search section to turn the indexing off if you find they’re getting in the way of your searches.