In that way it's like calling ERA theoretical. I mean, if a guy only throws 6 innings, and gives up 3 runs, his ERA is 4.50. Obviously, this is theoretical, because he only gave up 3 runs so how the heck could he possibly have an ERA of _4.5_????? Are you nuts????
So, the 3 runs are real runs. And the 6 innings are real innings. But the 4.5 is a theoretical projection based on a 9-inning opportunity.
Last I heard, no one has any issues with using this theoretical stat to judge the value of a pitcher.