Paragraph's height is different when html export and import are performed in the following way:

private void LoadRTB(RadRichTextBox RTB, string Txt)
{
    HtmlFormatProvider provider = new HtmlFormatProvider();
    HtmlImportSettings sett = new HtmlImportSettings();
    sett.UseDefaultStylesheetForFontProperties = true;
    provider.ImportSettings = sett;
    RTB.Document = provider.Import(Txt);
}

public string ExportHtml(RadRichTextBox RTB)
{
    HtmlFormatProvider provider = new HtmlFormatProvider();
    HtmlExportSettings sett = new HtmlExportSettings();
    sett.StylesExportMode = StylesExportMode.Inline;
    sett.DocumentExportLevel = DocumentExportLevel.Fragment;
    sett.ImageExportMode = ImageExportMode.UriSource;
    provider.ExportSettings = sett;
    return provider.Export(RTB.Document);
}

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.