Uhh, no fuel injected engine ever runs at the exact perfect mixture constantly. It is always swinging from rich to lean. When it's rich the O2 sensor reports that and the ECU leans the mixture, the O2 sensor reports that causing the mixture to be adjusted again, and so on... Ever wonder why Air/Fuel ratio gauges swing back and forth? (See pic below) The only time any engine will have a steady signal from an O2 sensor will be when it's either carbureted (non-feedback type) or when it's running in open-loop mode.
Note: Most cars will have a wider range than this illustration shows
