When sharing on Google Plus, Google will only use Schema if no The Open Graph protocol (OGP) is detected. In the event that no Schema or OGP is found then Google Plus will revert to the normal meta data that is used for ranking on Google Search. Obviously if none of these are found then Google will scrap text what it thinks is more ideal.
OGP is the standard format for setting page information which is then reflected when sharing on Social Media Platforms.
OGP looks something like this:
<meta property="og:title" content="..."/>
<meta property="og:image" content="..."/>
<meta property="og:description" content="..."/>
Here's some additional information about OGP that I answered awhile ago on Stack Overflow:
The Open Graph protocol is supported by a wide range of social
platforms which includes:
- Pinterest Developers
- Google Plus
Twitter however is singled out because they have decided to use their
own called Twitter Cards.
Twitter Cards look like this:
<meta name="twitter:card" content="summary" />
<meta name="twitter:site" content="@example" />
<meta name="twitter:creator" content="@example" />
<meta property="og:url" content="https://example.com" />
<meta property="og:title" content="Just an Example Title" />
<meta property="og:description" content="Example: Head, Shoulders, Knees and Toes! />
<meta property="og:image" content="http://example.com/knees-toes.jpg" />
If you want better control of what information appears when sharing articles on social media platforms then you should interment OGP and Twitter Cards to your website.