Hal 9000 said:
But then why does Palpatine seem to gloat as he describes Darth Plagueis not being able to save himself from being killed? Shouldn’t he just be Plagueis?
Palps: Take this! (Slices Plagueis)
I think you know the answer Hal, so does George Lucas when it comes to this whole topic. (Check out Clone Wars deleted scenes about the Mortis Arc.)