Pool A has three members. Why is the pool up?

Health monitoring being enabled on only one member means only that member is actively checked, but the pool's UP status still depends on whether that monitor succeeds, not merely whether it is enabled.

F5 BIG-IP evaluates pool availability based on a minimum number of active members, which defaults to one. When at least one pool member returns a successful health monitor response, the pool's status is marked as Available (green), allowing virtual servers associated with that pool to continue passing traffic.

Member and pool status in BIG-IP is determined by health monitor results, not by traffic volume.

If there is no health monitoring, members default to an Enabled state, but this choice does not explain why the pool is up in a scenario where monitoring is implied.