Although it is annoying in your case, this is well-known and intentional. A maven project should know about its execution directory only, no matter in what context it is executed.
I asked almost the same question: Maven variable for reactor root earlier, and the only answer that made sense was to use ${user.dir}, although it’s hacky and will not work if you build from a module directory.
(There is also this very verbose solution: Maven2 property that indicates the parent directory)