My background of AIX, '.Z' is used by the 'compress/uncompress' system commands and '.gz' is used with 'gzip/gunzip' system commands. Are you sure that the file wasn't created that way? 'compress' gets a 10% additional compression over 'gzip' and when disk drives were small, was a big deal. Today is not worth the CPU cycles.
OT: I've yet to see the file that compress crunches to a smaller size than gzip. Actually I thought for a long time (before I heard of the patents) that everyone had ditched compress for gzip because compress sucks so badly in comparison. Today, people burn a lot more CPU cycles using lzma, xz & Co. for a much better compression than either.