ROM might imply that it's a different hardware storage device or some such. That's not normally the case; it's just a different partition mounted at /system. Us

ROM might imply that it's a different hardware storage device or some such. That's not normally the case; it's just a different partition mounted at /system. User apps are in /data/app while system apps are in /system/app. /system shouldn't normally be writable at runtime on non-rooted devices, so updates get installed to /data/app like user apps. A proper uninstall with root, e.g. using Titanium Backup does remove the app from /system/app.