dns RR method is not equal balanced?

[Kay]

Dear my friends.

I use bind 8.4.7-REL on RHEL 4.4 OS and have thousands of domains.

In my case ;
some domain has 12 IPs but traffic of the server is not equal.
The traffic of 11 IPs is same and just 1 IP is higher than others.

Today, I moved the dns that is not equal to GSLB(F5) and set 
address-return 2(Maximum Addresses Returned).
And then, it's disappeared, equal traffic incoming completely.

Is there some kind of bugs in bind that I use?
or any idea?

Thanks.