Let's take a step back an look at the floc itself. In industrial system operating at a long sludge age bacterial floc has extracellular (EPS) materials and inorganics binding the bacterial matrix. If a long sludge age is coupled with quasi-toxic or inhibitory influent, the bacteria also excrete more EPS for protection. Key example of this is systems with periodic phenol and solvent loadings.
The EPS acts as a barrier between the encased bacteria and water. While the water may have 2 mg/L dissolved oxygen, inside the floc you may have much lower oxygen residuals. As the EPS layer becomes more "fixed" not necessarily larger - the bacteria in the core region of the floc can be in true anaerobic conditions. This can spur the growth of obligate anaerobes that should not normally exist in the aerobic biological treatment unit. Another feature of this phenomenon is the system with high DO having low DO filaments.
The oxygen barrier provided by EPS is also key to nutrient removal in systems utilizing anammox, aerobic granular sludge, and MBBR technologies.