Google calculates this 'right now' number as the number of unique users that have visited the site in the last 5 minutes. Similar to this, Ezoic calculates that number as any user that has some activity in the last five minutes. For the Ezoic definition, this activity could include scrolling down the page while reading, copy/pasting content, filling out information, etc.
Unfortunately, the Google definition of 'right now' does not accurately reflect active users on the site. Let's say a user comes to your site and they find a great page with a lot of interesting content and they take about 15 minutes to read the entire post. With the Google definition, they would mark that user as inactive after the first 5 minutes. That user would become active again, once they visit another page.
However, Ezoic's definition of real-time accounts for this! The system knows that the user is being active because the user is scrolling and copy/pasting the content along the way. Because of this difference, you will likely see a different number in Big Data Analytics real-time reports compared to Google Analytics.