Why does this function return a different value every time?

'(0 0 0) is a literal object, which is assumed to be a constant (albeit not protected from modification). So you're effectively modifying the same object every time. To create different objects at each function call use (list 0 0 0).

So unless you know, what you're doing, you should always use literal lists (like '(0 0 0)) only as constants.

On a side note, defining this function in the sbcl REPL you get the following warning:

  caught WARNING:
    Destructive function SB-KERNEL:%RPLACA called on constant data. 
    See also: 
      The ANSI Standard, Special Operator QUOTE 
      The ANSI Standard, Section 3.2.2.3

Which gives a good hint towards the problem at hand.

'(0 0 0) in code is literal data. Modifying this data has undefined behavior. Common Lisp implementations may not detect it at runtime (unless data is for example placed in some read-only memory space). But it can have undesirable effects.

• you see that this data may be (and often is) shared across various invocations of the same function

• one of the more subtle possible errors is this: Common Lisp has been defined with various optimizations which can be done by a compiler in mind. For example a compiler is allowed to reuse data:

Example:

(let ((a '(1 2 3))
      (b '(1 2 3)))
  (list a b))

In above code snippet the compiler may detect that the literal data of a and b is EQUAL. It may then have both variables point to the same literal data. Modifying it may work, but the change is visible from a and b.

Summary: Modification of literal data is a source of several subtle bugs. Avoid it if possible. Then you need to cons new data objects. Consing in general means the allocation of fresh, new data structures at runtime.