Chest Pull Rate

Hi,

please can you clarify the pull percentages on these chests that we have been seeing recently on the weekly events / SD and feud. Over the last prob 20+ I have never been able to pull one of the chase characters out of these chests, always ending up 1/2 to 2/3rds of the required shards, even though I have completed the event. On the flip side, I continually see the same people getting lucky on these shard chests and have a very good percentage. I do spend but just want to understand if these guys are just super lucky or is it based on spending more?