No you are not mistaken.

The prizes out of the magical sack are randomly sent. However if a script was added mistakes can be made and items not ment to be sent could be sent, (such as full sets and 3d items) and trying to work around this would be very time consuming. If a rework on the sack was made I would rather have the devs do a complete reset of how the sack works instead of just slapping a script on it and calling it a day. The sacks method of prize distribution has already be a topic of discussion even before this thread went up. For right now the current method is fine.