AGNC Investment has done a good job of creating value for investors, but there's a worrisome secret that you need to know before buying it.AGNC Investment (AGNC -0.39%) is not a dividend stock, despite the huge 14% or so dividend yield. It is a total return stock, which basically means to fully benefit here you need to reinvest dividends. But there's another open secret about AGNC's business model that you need to understand before you buy the mortgage real estate investment trust (REIT).What does AGNC Inve ...