To reproduce:
string text = "<p style=\"font-family:Calibri; font-size:15pt;\">Hi,<br/><br/><br/><br/>Regards<br/></p>";

HtmlFormatProvider provider = new HtmlFormatProvider();
RadDocument document = new RadDocument();
document = provider.Import(text);

radRichTextBox1.Document = document;

Click on an empty line and you will see that the font is different.