DEFVAR declares a variable to be special. That causes ALL uses of that variable to use dynamic binding. The function inside a LET with a dynamic variable does

DEFVAR declares a variable to be special. That causes ALL uses of that variable to use dynamic binding. The function inside a LET with a dynamic variable does not create a closure. If one calls CALC-WITHOUT-GAIN later, there is no binding - unless there is another dynamic binding of that variable by a different LET active.