Confluence does not currently support connecting to load-balanced AD servers.

http://confluence.atlassian.com/display/DOC/User+Management+Limitations+and+Recommendations
About half-way down:

"#3. Synchronising with AD servers behind a load balancer is not supported. As with synchronising between two different AD servers, Microsoft Active Directory does not replicate the uSNChanged attribute across instances. For that reason, we do not support connecting to different AD servers even when they are load balanced. You will need to select one server (preferably one that is local) to synchronise with instead of using the load balancer."

Many companies have a single DNS entry (mydomain.mycompany.com) that points to a load balancer that directs traffic to one of many directory servers. (Or, some companies might use DNS round robin.) However, neither the use of DNS round robin nor use of an actual load balancer is supported by Confluence – both approaches to load balancing direct traffic to different servers, which would violate limitation #3 above.
Currently, Confluence only supports connecting to a single directory server, not different (load-balanced) servers.