I am using WebGui version 6.2.0a but had the same problem with version 6.1.4 when I tried that one.
If I set the Value property (doesn't matter if in designer or programatically) to any value, the browser displays it at the minimum position. If I drag it with the mouse, the Value property is set to either minimum (for the left-most position) or maximum (for any other position). I don't get any other values.
Question is: Do I do something wrong? Is it a browser problem? For some reason the 'Catalog demo' showing trackbars works without a problem.