AC Current Measurement Deviation from Theoretical Calculation

Exactly correct Del, GA.

The root cause of this error can be anywhere from inaccurate loads, measurements, to even incorrect theoretical circuit analysis. The OP must calibrate their measurements to real reference standard devices.

The key for the OP to understand where the analysis error happened will be to go over the analysis method with somebody else and comparing the difference between measured and expected results. It is very easy and even common to forget that a design guide is actually an inaccurate approximation.

I have a few likely candidates for the OP.

1. Your single pole high pass filter should attenuate a 15.9 Hz by 3dB, but only at ∞ Hz will the attenuation be 0dB. Does your analysis include the expected attenuation at your power distribution frequency?

2. The high pass filter and every component in your signal path to your A/D will be loaded by the input impedance of the next circuit. Was this included in the analysis?

3. Was the amplifier gain calculated from the ideal amplifier equation with ∞open loop gain or from a real, precise finite amplifier gain equation?

4. All linear electronic parts are really only linear in a certain range of values. The compression you're seeing may just be an early indication of one part in the chain being too close to a signal maximum.

5. All parts but particularly capacitors have parasitic attributes that make them less than theoretical ideal parts.