I think group placing is based on your finishes in prior leaderboard events. Since this is your first one (?), I'm thinking they put you in a group with other players who are also doing their first LB event- which would be mostly new, low level players (and maybe some inactives or people who just dipped their toe into the game). Level seems to have little bearing on the groupings.
When the LBs first started, I noticed if I finished in first or second place I would get put in a hard group full of big spenders the next time. Or if I finshed low, I got put into an easy group the next time. That hasn't been happening the past few events. Instead, most of the people I'm grouped with play similarly to how I play (as far as gold and boost usage and total points scored at the end of the event). The groups don't feel quite as random anymore.
I think this means there have been enough LB events that S8 can compute useful, average LB scores for each player and use those averages to create the groups. Of course, this is just observation and speculation on my part. Makes sense, though.
I bet your placement will get more appropriate as you complete a LB or three and create an average LB score for S8 to work with (or maybe mean score is the better term?) I hope that makes sense.