The offset does not change the page number. It modifies the page which is referenced: the default 0 offset requests the attributes of the current page, therefore you have the current page number. An offset equal to -1 accesses the previous page; it returns the page number of the previous page (thus current page number - 1) if it exists otherwise the result is “void”.
Don’t fiddle with page offset. It is reserved for very special and specific uses.
What you need are page styles and special manual page breaks allowing you to start a page sequence at any number.