There are a couple of items in your rule configuration that could cause more traffic to go to one server or the other.
1. balance aca
If the css sees faster response times from one server to the extent that various thresholds are surpassed, the css will weight that server higher and distribute load accordingly. Switching your balance algorithm to round robin would arbitrarily balance the load amongst the servers.
2. advanced-balance sticky-srcip
As you probably know, this will "stick" individual clients to one server or another based on the IP address that they source their traffic from. It could be that you have a client or client(s) that happen to be stuck to a particular server that generate a unusually large volume of traffic. Some likely suspects in the internet for this type of traffic are mega-proxys like AOL where thousands of users appear under just a few IP addresses.
If your site requires that users stick to an individual server once they connect, but you still want to equalize load, you could try configuring stickyness based on a server or CSS supplied cookie.
