It is very common for LaserWriter registration to vary from page to page. Some print engines may have a larger degree of variation than others depending upon the design of the engine. Several variables have affect the registration and the difference between pages; paper alignment in the paper cassette, mechanical components of the printer itself, and imperfections in the paper. Also realize that it is next to impossible to align the pages exactly as they were when they went through the printer.
Also keep in mind that the LaserWriter Select family of printers were not designed for camera-ready artwork. The engine was designed for small business and home use. The transfer roller, for example, has a spring on each side to regulate the tension of the roller against the paper and the toner drum. As the printer operates, there could be a degree of variation from page to page. Also realize that small imperfections in the roller materials which can affect paper travel, as well as the paper alignment in the cassette. The paper, as it lies in the cassette, can be registered differently from one page to the next.
The 2 mm degree of registration difference is well within Apple's specifications for this print engine.