Michael had to know "it was Barzini all along," especially since his father came to that knowledge beforehand, and it was Barzini who invited Michael to the fateful meeting. So why did he ask Carlo? Perhaps a tactical move: Make Carlo think that by telling the truth, he'd be spared, and so he wouldn't put up a messy fight (or beg for his life) in the compound. Then he'd get in the car to the "airport," and Clemenza could garrot him, neat and clean.
Hell, what differenc