Different accounts are assigned to different shards. You can think of them as different database servers. Two different reasons. First is geographical region, they need to separate EU/U.K accounts from US accounts for some legal reasons. The second reason just has to do with processing speed. When they had too many accounts on one side, there were a lot of logjams. Things got a lot better when they added the second one.
All of this is really common with big companies. Most of them use multiple shards.
What’s unusual about SmartThings is that it will allow you to sign into the wrong Shard. Most companies automatically transfer you behind the scenes to the one your account is associated with.
My guess is it works this way because SmartThings didn’t realize they were going to have the problems and they added the second shard in a kind of emergency situation when a lot of things were broken and they just never redid the sign in process.