A single onion address is a single point of failure. If it is flooded, or comes under takedown pressure, or simply has a bad day on the Tor network, a market with only one address is unreachable and looks dead.
The pool
Awazon publishes several addresses that all resolve to the same market with the same account and balance. When one is under pressure, the others stay reachable and you rotate over. That redundancy is the entire reason a market with any sense runs more than one address.
Rotation
The pool changes over time as addresses are added and retired, each announced with a PGP signature. This is why you bookmark a verified reference rather than a raw address, so a rotation never strands you. See the URL page for the current pool.