The bit-depth depends upon how IM was compiled. On one machine it is probably 8-bit Q8 and the other 16-bit Q16. On the latter add -depth 8 to your command to force the result to 8-bits per color channel.
Good reading for newbies is:
http://www.imagemagick.org/script/comma ... essing.php
http://www.imagemagick.org/script/comma ... ptions.php
http://www.imagemagick.org/Usage/
http://www.imagemagick.org/script/advan ... lation.php