The Mysterious Case Of The Vanishing Bull Volume

Some volumetric observations: the chart below demonstrates a "cumulative volume divergence from average" histogram. Recently, on up days, the volume has been significantly below average, and yet accelerates above average on down days. Another observation: on down days, the VWAP for the SPY is the magical barrier that simply refuses to be breached. Today for example, the ramp into the close occurred despite that the volume pushing the SPY higher was again below average toward the end of the day.