Question about Z8's

i've been bored all morning and looking up exotic cars came across the Z8 and this one thing struck me as interesting the Z8 has the BMW M5 motor that says M power on it with the huge ducks and crap, however if you get the Automatic version of the Z8 you get a little bit of a beefed up 540i motor. Anyone every see this or know why they would do it since you can hook up a SMG to that M5 motor no problem.