This blogpost consist of everything you need about the topic https://blog.rollbit.com/rollbit-rewards-breakdown/
This site is quite popular even before they make their way into bitcointalk. They have partnered with few streamers / youtubers in the past to get more exposures to their site which means they are pretty serious to develop even further
This kind of multipliers dont appear that often. Probably something 1 in a mil or 100k
Most of the game in the site are 3rd party slots which is not provably fair but it doesnt really matter. This is the new period for bitcoin gambling industry anyway, most sites even offer these kind of slots nowadays instead of some original provably fair game