A good analogy is what happens when you create a guard with defguard, under the hood it creates two versions of the Macro. one that is executed when is called into the guard context, so it is really limited what you can do there, So no variable assignments is allowed,
so if you want to call two different functions on the same argument, you make two copies of it (similar to what you do in your initial example),
but if the macro is called out of the guard context, is it optimized, so each argument is assigned to a variable, and then it is referred to it, as in my version of your code.
