Pool round-robin load balancing not balanced

Depending on your LTM CMP may be responsible. see SOL7751 Scenario 2: Virtual server with CMP enabled on a BIG-IP 8800

Four connections are made to the virtual server, unlike the first scenario where CMP was disabled, the BIG-IP distributes the connections across the multiple TMM processes. The BIG-IP 8800 with CMP enabled can use four TMM processes. Since each TMM handles load balancing independently of the other TMM processes, it is possible that all four connections are directed to the same pool member.

--Connection 1--> | | --Connection 1--> TMM0 --> 10.0.0.1:80 --Connection 2--> |-> BIG-IP Virtual Server ->| --Connection 2--> TMM1 --> 10.0.0.1:80 --Connection 3--> | | --Connection 3--> TMM2 --> 10.0.0.1:80 --Connection 4--> | | --Connection 4--> TMM3 --> 10.0.0.1:80

The CMP feature is designed to speed up connection handling by distributing connections across multiple TMM processes. While initially this behavior may appear to favor one or several servers, over time the load will be distributed equally across all servers.