Are your aeration basin D.O. meters telling you the whole D.O. story?

D.O. meters report dissolved oxygen in the water phase of the aeration basin.  We know that in a flask, obligate aerobes such as AOB do very well at 2.0 mg/L DO.  Why is it that in many aeration basins the DO must be at least 3.5 mg/L for proper obligate aerobic organism growth?

I have seen two reasons why you may need to run higher than textbook D.O. residuals.

• DO probe placement – a single probe can have sampling error and can easily be checked by testing other points in the basin with a portable meter.
  
  
• DO in floc is different than in the water phase (where the probe is located) - floc is composed of microbes, EPS, non-soluble organics, and inorganics.  DO must penetrate the floc to reach microbes deep inside the floc.  Larger, higher EPS flocs can have anoxic/anaerobic zones inside.
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Do you have any other ideas why many systems require operation above 2.0 mg/L D.O. residuals in the aeration basin?