
Originally Posted by
Viena
It probably works the following way.
The majority of the BCNM areas have 3 replicas of the arena where you fight, this is also Diabolo's area case. Those replicas exist to allow up to 3 groups to fight simultaneously and reducing possible wait-time (although from a programming standpoint it's not the most clever way to provide instanced content).
I wouldn't be surprised if when you enter you automatically get assigned arena #1 in case it's free, or assigned the next one that is if it wasn't. That would pretty much explain it, as in Diabolo's area, each of the 3 arenas holds a different tile pattern.