Sometimes, when creating a tar-ball on MacOSX, you end up having two files for each file you add to the archive: the file itself and one with ._ in front of the name.
Apparentely, that’s how MacOSX stores the meta-data of the file for file-systems without meta-data features (anything other than HFS+ I presume?). Basically, setting the environment variable
COPY_EXTENDED_ATTRIBUTES_DISABLE=true disables this feature. More info might be here.