I thought that is what it did, this is why I asked if order matters. But that would be really wrong design if this was the case. Also help says
And dictionary says that simultaneous means: occurring, operating, or done at the same time.
So it should be doing the substitution in "batch mode", ie, at same time. Not do one. Evaluate the new expression, and now do the next replacement on the new expression, which ofcourse could now have changed, reshuffled, order of operands might change, number of operands changed, and so on. This design would make no sense from the user point of view if this was the case, right?
But who knows. May be help is wrong and may be it works as you said.