The "Online Readership" total metric is the sum of the site-wide monthly visitors of all online publications from your coverage.
If you want to get an idea of the number of impressions the articles you added as online coverage received then please see the "estimated coverage views" metric.
My online readership number is not totalling up properly?
Sometimes coverage is featured from the same domain multiple times in a single book.
e.g. You might have a book with 2 bits of coverage in, both on theguardian.com
The Guardian had 271 million visitors in a month (at the time of writing this post).
Just because you had 2 separate mentions in The Guardian, the fact remains that only 271 million people visit (all the pages of) the Guardian. So that means that the Online Readership will be 271 million.
So you could have 10 different bits of coverage in the Guardian. But we’d only ever add 271 million people to the total. Not times that by 10.