Everybody gets placed in a shard with people when they first join.
No one ever leaves that shard. No one ever joins that shard once a few weeks have passed. You are competing against the same people you were competing against when you first started playing this game, and those will be the only people you ever compete against.
The time zone change only let people change what time they get paid out at.
The only shard shuffle they've ever done is that Ship shard shuffle for everyone who was unlucky enough to have logged in right away after ships were released and ended up in that one super active fleet shard.
