In looking at the AHK Help file it states that the X,Y can be expressions, but it doesn't say that they can be variables. Maybe for the X,Y on tooltip it doesn't recognize variables.....
The first number, 2000 in this case is the left right and the second number, 20, is the up down. You can also add this line right below that line to get a "tray tip" when you hover the mouse over the icon in the tray: