Yeah good point  - weird how I've never noticed before though. I just did a little test in the car park at work, wouldn't say the slope to the next floor is majorly steep but the Zed even began to roll back here, infact I woulod say unless I applied the brakes the car wouldn't of had a behind left! 
That is odd as i dont think it should roll too much on a little slope.  It should be idling and going forward like a manual in first gear, so if you leave it without and throttle, you should advance forward at about 5/7mph.  
Maybe your engine isnt idling propery and needs a service? 
  
(btw - dont get too scared as i dont really know what i am saying as its really early and i have a hangover )