GUI Slider sets a value but the value appears to be reset by the end of OnGUI()

The following code always prints "1.0":

private var someVal=1.0;
function OnGUI() {
    someVal=GUI.HorizontalSlider(Rect(10,10,200,50),someVal,0.0,1.0);
}
function Update() {
    print(someVal);
}

The following code always prints the correct value which I define by sliding the slider:

private var someVal=1.0;
function OnGUI() {
    someVal=GUI.HorizontalSlider(Rect(10,10,200,50),someVal,0.0,1.0);
    print(someVal);
}

Additionally, I don't know if this is intentional or not, but when I slide the slider, I see a ghost version of the little knob still in its original position, which I guess is intentional, but when I release the mouse button it remains there.
Obviously I want my someVal to get adjusted with the slider. Does someone know what I'm doing wrong here?