MVC HtmlHelper: why two calls gives different output?

I've written a HtmlHelper extension method which reads content of CSS file and wraps it into tag. See the code below:

    public static IHtmlString EmbedCss(this HtmlHelper htmlHelper, string cssFile)
    {
        var httpContext = htmlHelper.ViewContext.RequestContext.HttpContext;
        var physicalCssFileName = httpContext.Server.MapPath(cssFile);
        if (!File.Exists(physicalCssFileName))
        {
            throw new FileNotFoundException(string.Format("Requested CSS file \"{0}\" is not found", physicalCssFileName));
        }
        var css = File.ReadAllText(physicalCssFileName);
        var tb = new TagBuilder("style");
        tb.Attributes["type"] = "text/css";
        tb.SetInnerText(css.TrimStart(' ').TrimEnd(' '));
        return MvcHtmlString.Create(tb.ToString());
    }

Then I call this method in a view:

@Html.EmbedCss("~/assets/css/critical.css")

The quote and double-quoted characters in the output are being encoded as $#39; and $quot; respectively. If I replace the call of the extension method with the following code the quotation characters in the output aren't being encoded.

<style>@Html.Raw(File.ReadAllText(Server.MapPath(("~/assets/css/critical.css"))))</style>

Could anybody explain why it happens?