Question: I have runnning e-commerce based ruby on rails application on AWS stack. I am running ubuntu 10.04 ec2 instances with Elastic load balancer and I have maintained equal number of instance in the both the availability zone, 1a and 1b. But according to my observation, ELB seeming to be pushing more traffic to 1a rather then dividing it equally. Though the health of the instances running in 1b is good and also I have disabled the sticky session on the ELB. I have 2 large and 1 medium instances running on both the availability zones. 

What the cause of in equal distribution of the load.

Answer: 
In my experience, this can happen if a disproportionate amount of traffic is coming from a single network or ip address. 

ELB uses different layers of balancing. DNS load balancing will send it to a set of IP addresses in one of the two zones, and the software load balancer will distribute traffic between instances in the zone.

If you have a lot of traffic coming from the same network, its likely that a lot of users are getting the same DNS resolution on your load balancer and ending up in the same zone.
If the source traffic is coming from a single Network/IP range or IP address, ELB might load balance the traffic disproportionately to the backend. I have discussed this point as well as couple of other details to note on ELB in my blog "Dissecting ELB". I have also noticed this behavior in some popular OSS LB implementations as well, You can have balance algorithm as "source" and as well as session sticky combined . If session ID is not sent on the HTTP, then it will load balance based "source".