Do you know of any lv 90 55k*+ player in your server cluster that is in a different sage?
If so, something fishy is going on.
If not, it's simply a case of... there aren't enough high power player who signed up for sage in your server cluster, so everyone 55k and up got stuffed into one, including the 75k. (I would assume a grouping that includes 75k is the first grouping of the server cluster, because otherwise it would imply there are enough 75k+ for their own grouping, which is kinda scary)
Remember, the "promised" grouping for group 1 is effectively "the N highest power player who are level 90 from the server cluster" where N is the size of the sage battle. For many clusters, that's going to be somewhere around 60k to 120k(or whatever highest power is in that cluster)
Btw, I assume that the server cluster is the same as the space time battle one, thou I can't be 100% certain.
55k+*: to be more specific, players who have a higher "max power" than your "max power"(since you are the "55k" here), where max power is the power that a player have on the leader board which I assume the sage sorts player by.