I just made them consistent. It certainly helps me visually. Also a list implies you intend to evaluate it by executing a function. A vector implies you do not intend to call a function.
def
foo(x)
bar(x)
end
(defun averagenum (n1 n2 n3 n4)
(/ (+ n1 n2 n3 n4)
4))
DEFSOMETHING name arglist
body
Lisp programming is more about thinking of trees of code and their possible manipulation - even independent of a visual notation and especially independent of the exact delimiter used.
The delimiters are in shape recognition much less important than the shape itself.
I wrote it that way so it's easier for non-lisp programmers to compare with what they're more used to as well.