Sink vertices (those with no outgoing edges) should evenly distribute their rank to the entire graph but in the current implementation it is just lost.
In the original form of PageRank, the sum of PageRank over all pages was the total number of pages on the web at that time
I found the issue, while comparing results with the spark graphX.
So this is a copy of https://issues.apache.org/jira/browse/SPARK-18847
How to reproduce:
They fixed the issue by normalising values after each step.
The other way to fix is to send the message to it self (stay on the same page).
To workaround the problem just add self pointing edges:
Then you'll get always correct sum. But I'm not sure it is a proper assumption.