User wants perfect 'Round Robin' load distribution

+1 for WLB's explanation. It might be that the load balancing itself is even, but that actual sessions are greater or lesser on different systems based on what the user is doing. Try this iRule and see what you get:

when LB_SELECTED {    
    log local0. [LB::server]
}

It's going to report the chosen server in every load balancing event. Do you see even distribution with round robin? Least Connections is a good way to temper that, but then you'll never really attain a perfect balance across all of the servers as long as the data flowing across the sessions (what the user is asking for) can be different.