Cabal sandbox and rebuild of added sources

Hi, I just tried the sandbox functionality of cabal with the current version on github (b89d894947a9ec6fde78ebc6e7f6978f0b117eb1). If the source added by 'cabal sandbox-add-source' changes, than it gets rebuild by calling 'cabal sandbox-build', but not if I'm just calling 'cabal sandbox-install'. Is there a reason for this behavior? Greetings, Daniel