Are you sure that term represents a percentage at all? The way I think how weight would work is you have an empty urn and for every event with a weight 70 you put 70 balls with a unique color into the urn. And for Assassination event you put 100 black balls into the urns. Then whenever a mega-event is called up by the game, you simply pick a ball out of the urn. That the number is 100 is just just coincidence, they could have gone with a 150 or 360 weight or whatever else they wanted, it's just a relative measure of the chance of one event happening as compared to other events.