SetCtrlVal() with textbox - function does not work as intended?

I use this command to update a textbox with a timer:    

SetCtrlVal   (MyPanel,  MyPanel_MyTextBox,  SomeText);

It keeps adding 'SomeText' into MyTextBox instead of setting it with that value.  It only works as I want when I use:

ResetTextBox   (MyPanel,  MyPanel_MyTextBox,  SomeText);

Is this supposed to be the way SetCtrlVal() work, because that is how I understand it?