AlumniJacqueline,
Windows changes the button gradient a little when the mouse cursor is over the button. Perhaps, this causes differences. Please check whether the mouse cursor is over or off the button. There can also be other reasons, like the OS color scheme or color depth. Please see Factors Affecting Image Comparison in TestComplete help.
To exclude the problematic regions from the comparison, you can fill them with some color considered "transparent" or create masks.
See Comparing and Finding Images - Specific Tasks.
