UPDATE: currently, aka 27 Dec 2015, there’s an issue opened on their tracker regarding this feature/bug
The same blog you’re indicating, states the following
In addition, the text is center-aligned!
This got me thinking, so I played a bit with the right margin features and, as you can see in the pictures below, the 2 seem related:
1. Right margin at 80 I have it at 120 and I can’t recall the default value, but this looks like yours.
2. Right margin at 160
In conclusion, the larger the number, the smaller the right margin, and in addition so to speak, the smaller the left margin.