"matt b" has it right, but to be specific, the "install" goal copies your built target to the local repository on your file system; useful for small changes across projects not currently meant for the full group.
The "deploy" goal uploads it to your shared repository for when your work is finished, and then can be shared by other people who require it for their project.
In your case, it seems that "install" is used to make the management of the deployment easier since CI's local repo is the shared repo. If CI was on another box, it would have to use the "deploy" goal.