It's because you are actually concatenating two strings. Please note that you call ToString() on the second value. Try this:
txtEstbFee.Text = (Convert.ToDouble(dt.Rows[0]["EstbFee"]) + Convert.ToDouble(dt.Rows[0]["FutureEstbFee"])).ToString().Trim();
Or better break into several steps:
var estbFee = Convert.ToDouble(dt.Rows[0]["EstbFee"]);
var futureEstbFee = Convert.ToDouble(dt.Rows[0]["FutureEstbFee"]);
var totalFee = estbFee + futureEstbFee;
txtEstbFee.Text = totalFee.ToString();
This way it's easy to read.