But why would he let Zack know that it wasn't Cloud so much in the body? After all knowing Zack, he'll try to make Sephiroth leave it. Or at least make him give full control back to Cloud. It's -Cloud's- body, after all.

And, well, Sephiroth kinda likes the idea of having a little mental pet, who he could do whatever he wanted to, and nobody could stop him....