For example, if the document has "Times New Roman" as a default font family, the "TableNormal" style doesn't have set that property and there is a Table element in the header - the value is not properly obtained ("Verdana" is applied, which is the default value for that Span property).
The problem is observed for all default span style properties.

Workaround: Change the table style by setting the necessary properties from the default style. For example for the font family:

object documentFontFamilyPropertyValue = this.radRichTextBox.Document.Style.GetPropertyValue(Span.FontFamilyProperty);
foreach (StyleDefinition tableStyle in this.radRichTextBox.Document.StyleRepository.Where(s => s.Type == StyleType.Table && s.BasedOn == null))
{
    StylePropertyBase styleProperty = tableStyle.GetProperty(Span.FontFamilyProperty);
    if (styleProperty.ValueSource == RadValueSource.Default)
    {
        tableStyle.SetPropertyValue(Span.FontFamilyProperty, documentFontFamilyPropertyValue);
    }
}