One would think that a general standard has to be more liberal than UL requirements. In reality, for V>150V IPC actually calls for larger spacings between uncoated external conductors than those you can derive from UL 60950-1 Table 5B in conjunction with the 40V/mil criterion. Note that generally all IPC doc's are voluntarily rather then mandatory. Particularly, they state that "Existence of such Standards and Publications shall not in any respect preclude any member or nonmember of IPC from manufacturing or selling products not conforming to such Standards and Publication".