It seems the height of the line is not properly calculated for specific fonts which leads to difference in the way the documents look. Document with one full page of text formatted with Arial Unicode MS looks shorter in RadRichTextBox.
The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.