symbol-function vs defun question

Greetings.

I have been puzzling over a difference between using SYMBOL-FUNCTION
and DEFUN within a let binding.  Here's a contrived example that
illustrates:

(let ((table1 (make-hash-table)) (table2 '(1 2 3)))
  (setf (symbol-function (intern "DESCRIBE-SOME-TABLES"))
        (lambda () (print (list :table1 table1 :table2 table2))))
  (defun a-function () (describe-some-tables)))

When this is compiled the first time, the compiler issues the warning:

;   Undefined function:
;     DESCRIBE-SOME-TABLES
;   caught 1 STYLE-WARNING condition

Yet the function works:

(a-function)
;; => (:TABLE1 #<HASH-TABLE :TEST EQL :COUNT 0 {1003B80DA3}> :TABLE2 (1 2 3))

Compiled subsequent times the warning is gone - presumably as the package namespace now knows about the function. But unbind it:

(fmakunbound 'describe-some-tables)

and recompile and the warning is back the first time.

Yet if I use DEFUN there is no such warning:

(let ((table1 (make-hash-table)) (table2 '(1 2 3)))
  (defun describe-some-tables ()
    (print (list :table1 table1 :table2 table2)))
  (defun a-function () (describe-some-tables)))

Why is this? And is there a way to get around it so I can define the
function using symbol-function? The reason for doing this is to be
able to define functions within macros whose names are composed from
its arguments.

Thanks.

-- Jeff