Bamboo agents build to the revision that was latest at the time the build entered the queue.
If the build festers in the queue for a while and more commits are made then the agent should build to the revision at the time the build was taken from the queue.

Eventually bamboo will run a build with the latest revision because it will detect the more recent changes.

However users should have an option to choose 'Checkout to latest regardless' and force the agent to update to the latest revision.