Have you installed a sketch that uses that AC reference?

If it is apparent power that you are reading rather than real power, it doesn’t know the direction of flow, it only knows the magnatude of the current and multiplies that by the defined voltage, which is positive x positive = positive values.

ok, so I googled and found that With the addition of an AC-AC adapter to provide an AC voltage sample the shield can monitor real power, AC RMS voltage and power factor.

I get it how an AC-AC adapter will let me know the AC voltage, but how does it help with real power. How does it tell the direction of the flow?

The position of the waveforms relative to each other. Assuming a pure resistive load (PF 1.0) in one (forward?) direction the voltage and current waveforms will be aligned and in the other (reverse?) direction the waveforms will be at 180° to each other. From this the sketch can determine the direction.