If you are dealing with a later model engine with an Air Fuel sensor, you can keep the bar in the middle, or anywhere else the PCM wants because the sensor can tell the PCM exactly what the fuel/air mix is. The PCM could target a 20:1 lean mix and the Air Fuel sensor can feedback that information and tell the PCM when it got there. In this way, the PCM can achieve any desired fuel ratio for any driving condition.
Before these advanced sensors, the technology just wasn't there. As I mentioned before, the older style Zirconia sensors didn't have the capability to tell the PCM where the mixture was other than it was rich or lean. All the PCM can do with that information is cut fuel when rich and add fuel when lean, bouncing back and forth.
If you had a disconnected injector with an older style O2, then this would tend to lower the voltage the O2 reports (lean indication). The PCM may then respond to what it believes is a general lean condition and add fuel..this would make the remaining cylinders too rich.
Hope this helps.
(The Picotologist) http://www.autonerdz.com
THE PicoScope Automotive Authority
In North America