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But you are measuring only 50 W! the maximum you can measure is 25 kW, so you are below the range where the measurement is accurate. Even if you have calibrated your Arduino accurately, the current transformers are only guaranteed to be accurate to within ±3% above 10 A (≈ 2300 W). Below about 200 W, electrical noise in the measuring circuits will add to the current you measure, and it is likely to be that, which is causing the positive values to be larger than the negative
If you measure a much bigger load, I think you will find that the positive and negative values will come much closer, as a percentage.
The calibration instructions are in the ‘Learn’ section. Calibration will correct the errors caused by tolerances in the manufacturing of all the different components.