If you plugged a mouse device into your machine, which was previously installed before you configured your controls, when you plug that device in again, it will shift the numbering upwards. The same type of thing can also happen as a result of plugging one or more of the mouse devices into a USB port other than where it resided when the controls were configured.
This is a Windows USB handling thing that has nothing to do with the EI Trackball. The same thing can mess with the ordering of USB gamepads, etc. One way to keep it from happening is to re-configure your software to reflect the controls' current arrangement and don't remove any of them, or completely uninstall whichever device is causing the shift upward and re-install it while the other devices are plugged in and in the order you want them to be.
RandyT