This is a "by design" behavior. FORM is, by default, considered as an empty HTML element. The reasons are explained here on SO (check my answer): HtmlAgilityPack -- Does <form> close itself for some reason?

But this is also configurable, you just need to instruct the parser to behave differently, like this:

HtmlAgilityPack.HtmlDocument hChart = new HtmlAgilityPack.HtmlDocument();

// remove all specific behaviors for the `FORM` element
HtmlAgilityPack.HtmlNode.ElementsFlags.Remove("form");

hChart.LoadHtml(postRequest("http://www.entropedia.info/Chart.aspx?chart=Chart", "ctl00%24ContentPlaceHolder1%24DG1%24ctl19%24ctl05"));
HtmlNodeCollection chartStrings = hChart.DocumentNode.SelectNodes("/");