Because parameter lists in Arc support destructuring. What that means is that anywhere you can write variable to bind a name to a value (such as in (let variable 10 (prn variable))), you can also write (v1 v2) to bind a list's first element to v1 and second element to v2. And (o v default) denotes optional parameters. Perhaps some examples would be clearer:
(with (i 1 v 5 x 10)
(let a (list i v) (prn "a is (1 5)."))
(let (b c) (list i v) (prn "b is 1 and c is 5."))
(let (d . e) (list i v x) (prn "d is 1 and e is (5 10)."))
(let (f (o g)) (list i) (prn "f is 1 and g is nil."))
(let (h (o j 42)) (list i) (prn "h is 1 and j is 42."))
(let (k (o m)) (list i v) (prn "k is 1 and m is 5."))
(let (n (o p 42)) (list i v) (prn "n is 1 and p is 5.")))
defm adds a (t var type) syntax; if you left out the t, you would have ordinary destructuring bind.
